Concentration is an expression of how muchsoluteis dissolved in asolventin a chemicalsolution. There are multiple units of concentration. Which unit you use depends on how you intend to use the chemical solution. The most common units are molarity, molality, normality, mass percent, volume percent, and mole fraction. Here are step-by-step directions for calculating concentration, with examples showing the math and tips on when to use the units.
How to Calculate Molarity of a Chemical Solution
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Molarity is one of the most common units of concentration. It is used when the temperature of an experiment won't change. It's one of the easiest units to calculate. You get the mass of solute for the solution, mix the solute with a known volume of solvent, and divide mass by volume for concentration.
Calculate Molarity: moles solute per liter of solution (not volume of solvent added since the solute takes up some space)
symbol: M
M = moles / liter
Example: What is the molarity of a solution of 6 grams of NaCl (~1 teaspoon of table salt) dissolved in 500 milliliters of water?
First, convert grams of NaCl to moles of NaCl.
From the periodic table:
- Na = 23.0 g/mol
- Cl = 35.5 g/mol
- NaCl = 23.0 g/mol + 35.5 g/mol = 58.5 g/mol
- Total number of moles = (1 mole / 58.5 g) * 6 g = 0.62 moles
Now determine moles per liter of solution:
M = 0.62 moles NaCl / 0.50 liter solution = 1.2 M solution (1.2 molar solution)
Note that I assumed dissolving the 6 grams of salt did not appreciably affect the volume of the solution. When you prepare a molar solution, avoid this problem by adding solvent to your solute to reach a specific volume.
How to Calculate Molality of a Solution
Molality is used to express the concentration of a solution when you are performing experiments that involve temperature changes or are working with colligative properties. Note that with aqueous solutions at room temperature, the density of water is approximately 1 kg/L, so M and m are nearly the same.
Calculate Molality: moles solute per kilogram solvent
symbol: m
m = moles / kilogram
Example: What is the molality ofa solution of 3 grams of KCl (potassium chloride) in 250 ml of water?
First, determine how many moles are present in 3 grams of KCl. Start by looking up the number of grams per mole of potassium and chlorine on a periodic table. Then add them together to get the grams per mole for KCl.
- K = 39.1 g/mol
- Cl = 35.5 g/mol
- KCl = 39.1 + 35.5 = 74.6 g/mol
For 3 grams of KCl, the number of moles is:
(1 mole / 74.6 g) * 3 grams = 3 / 74.6 = 0.040 moles
Express this as moles per kilogram solution. Now, you have 250 ml of water, which is about 250 g of water (assuming a density of 1 g/ml), but you also have 3 grams of solute, so the total mass of the solution is closer to 253 grams than 250. Using 2 significant figures, it's the same thing. If you have more precise measurements, don't forget to include the mass of solute in your calculation!
- 250 g = 0.25 kg
- m = 0.040 moles / 0.25 kg = 0.16 m KCl (0.16 molal solution)
How to Calculate Normality of a Chemical Solution
Normality is similar to molarity, except it expresses the number of active grams of a solute per liter of solution. This is the gram equivalent weight of solute per liter of solution.
Normality is often used in acid-base reactions or when dealing with acids or bases.
Calculate Normality: grams active solute per liter of solution
symbol: N
Example: For acid-base reactions, what would be the normality of 1 M solution of sulfuric acid (H2SO4) in water?
Sulfuric acid is a strong acid that completely dissociates into its ions, H+ and SO42-, in aqueous solution. You know there are 2 moles of H+ ions (the active chemical species in an acid-base reaction) for every 1 mole of sulfuric acid because of the subscript in the chemical formula. So, a 1 M solution of sulfuric acid would be a 2 N (2 normal) solution.
How to Calculate Mass Percent Concentration of a Solution
Mass percent composition (also called mass percent or percent composition) is the easiest way to express the concentration of a solution because no unit conversions are required. Simply use a scale to measure the mass of the solute and the final solution and express the ratio as a percentage. Remember, the sum of all percentages of components in a solution must add up to 100%
Mass percent is used for all sorts of solutions but is particularly useful when dealing with mixtures of solids or anytime physical properties of the solution are more important than chemical properties.
Calculate Mass Percent: mass solute divided by mass final solution multiplied by 100%
symbol: %
Example: The alloy Nichrome consists of 75% nickel, 12% iron, 11% chromium, 2% manganese, by mass. If you have 250 grams of nichrome, how much iron do you have?
Because the concentration is a percent, you know a 100-gram sample would contain 12 grams of iron. You can set this up as an equation and solve for the unknown "x":
12 g iron / 100 g sample = x g iron / 250 g sample
Cross-multiply and divide:
x= (12 x 250) / 100 = 30 grams of iron
How to Calculate Volume Percent Concentration of a Solution
Volume percent is the volume of solute per volume of solution. This unit is used when mixing together volumes of two solutions to prepare a new solution. When you mix solutions, the volumes aren't always additive, so volume percent is a good way to express concentration. The solute is the liquid present in a smaller amount, while the solution is the liquid present in a larger amount.
Calculate Volume Percent: volume of solute per volume of solution (not volume of solvent), multiplied by 100%
symbol: v/v %
v/v % = liters/liters x 100% or milliliters/milliliters x 100% (doesn't matter what units of volume you use as long as they are the same for solute and solution)
Example: What is the volume percent of ethanol if you dilute 5.0 milliliters of ethanol with water to obtain a 75-milliliter solution?
v/v % = 5.0 ml alcohol / 75 ml solution x 100% = 6.7% ethanol solution, by volume.
How to Calculate Mole Fraction of a Solution
Mole fractionor molar fraction is the number of moles of one component of a solution divided by the total number of moles of all chemical species. The sum of all mole fractions adds up to 1. Note that moles cancel out when calculating mole fraction, so it is a unitless value. Note some people express mole fraction as a percent (not common). When this is done, the mole fraction is multiplied by 100%.
symbol: X or thelower-case Greek letter chi, χ, which is often written as a subscript
Calculate Mole Fraction: XA = (moles of A) / (moles of A + moles of B + moles of C...)
Example: Determine the mole fraction of NaCl in a solution in which 0.10 moles of the salt is dissolved in 100 grams of water.
The moles of NaCl is provided, but you still need the number of moles of water, H2O. Start by calculating the number of moles in one gram of water, using periodic table data for hydrogen and oxygen:
- H = 1.01 g/mol
- O = 16.00 g/mol
- H2O = 2 + 16 = 18 g/mol(look at the subscript to note there are 2 hydrogen atoms)
Use this value to convert the total number of grams of water into moles:
(1 mol / 18 g ) * 100 g = 5.56 moles of water
Now you have the information needed to calculate mole fraction.
- Xsalt = moles salt / (moles salt + moles water)
- Xsalt = 0.10 mol / (0.10 + 5.56 mol)
- Xsalt = 0.02
More Ways to Calculate and Express Concentration
There are other easy ways to express the concentration of a chemical solution. Parts per million and parts per billion are used primarily for extremely dilute solutions.
g/L = grams per liter = mass of solute / volume of solution
F = formality = formula weight units per liter of solution
ppm = parts per million = ratio of parts of solute per 1 million parts of the solution
ppb = parts per billion = ratio of parts of solute per 1 billion parts of the solution.
FAQs
What are three ways to measure the concentration of a solution? ›
- Molarity (M): It is calculated by the number of moles (n) of solute and volume of solution (V). ...
- Mole fraction (x): It is the ratio of number of moles of particular element ( ) and total number of moles of substance ( ).
- concentration. how much solute is dissolved in a given amount of solution.
- (weight/volume)% number of grams of solute dissolved in 100 mL of solution.
- (w/v)% = mass of solute(g)/volume of solution (mL) *100% ...
- volumetric flask. ...
- (volume/volume)% ...
- (v/v)% = ...
- concentration conversion factors. ...
- parts per million.
The equation for Beer's law is a straight line with the general form of y = mx +b. where the slope, m, is equal to εl. In this case, use the absorbance found for your unknown, along with the slope of your best fit line, to determine c, the concentration of the unknown solution.
How do I calculate the concentration? ›The standard formula is C = m/V, where C is the concentration, m is the mass of the solute dissolved, and V is the total volume of the solution.
Which is the best method to measure the concentration of the solution? ›a. Molarity: It is the number of moles of solute contained in one liter of solution. It is a commonly used method for measuring the concentrations.
How can you determine the concentration of each solution? ›Just divide moles of solute by volume of solution.
What is method of concentration of solution? ›Molarity (M):
One of the most commonly used methods for expressing the concentrations is molarity. It is the number of moles of solute dissolved in one litre of a solution.
(a) Concentration of a solution is defined as the amount of solute that is present in a given amount of solution. It can be expressed in terms of: Mass by the mass percentage of a solution =mass of solutemass of solution×100.
How do you find the concentration of an unknown concentrated acid? ›The concentration of an acid solution can be determined by titration with a strong base. First, calculate the number of moles of strong base required to reach the equivalence point of the titration. Then, using the mole ratio from the balanced neutralization equation, convert from moles of strong base to moles of acid.
How do you find the concentration of an unknown solution using a standard curve? ›To calculate the sample concentration based on the standard curve, first you find the concentration for each sample absorbance on the standard curve; then you multiply the concentration by the dilution factor for each sample.
What is concentration in chemistry? ›
The concentration of a substance is the quantity of solute present in a given quantity of solution. Concentrations are usually expressed in terms of molarity, defined as the number of moles of solute in 1 L of solution.
How do you find concentration and dilution? ›Using C1V1 = C2V
To make a fixed amount of a dilute solution from a stock solution, you can use the formula: C1V1 = C2V2 where: V1 = Volume of stock solution needed to make the new solution. C1 = Concentration of stock solution. V2 = Final volume of new solution.
Chemists can express concentrations in various ways including: Molarity (M), Parts per million (ppm), % composition, or gram/Liter (g/L).
What are the three 3 ways to measure the concentration of a solution and differentiate one from the other? ›This percentage can be determined in one of three ways: (1) the mass of the solute divided by the mass of solution, (2) the volume of the solute divided by the volume of the solution, or (3) the mass of the solute divided by the volume of the solution.
Which is the most common measure of concentration used in chemistry? ›By far the most common concentration term for a chemist is molarity. It is the ratio of moles of solute to the overall volume of the solution in liters.
What are the six ways in calculating the concentration of solution? ›- Percentage by weight (w / w %)
- Percentage by volume (V / V%)
- Weight by volume (w / v%)
- Mole fraction (x)
- Parts per million (ppm)
- Molarity (M)
- Molality (m)
- Normality (N)
The simplest way to describe the concentration of solution is mass by mass and mass by volume.
What is the concentration of a solution measured in? ›The concentration of a solution is a measure of the amount of solid dissolved in 1 dm 3 of solution. When the mass of solute dissolved is measured in g, the units for concentration are g/dm 3.
What is concentration of solution and how it is expressed? ›The concentration of the solution is the amount of solute present in a given amount of solvent. The concentration of a solution is expressed in the following ways: Mass by mass percentage of a solution = Mass of solutionMass of solute×100.
What is the total concentration in a solution called? ›Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of solution.
How do you find the concentration of a solution given molarity? ›
To calculate the Molar Concentration, we will find the molar concentration by dividing the moles by liters of water used in the solution. For example, the acetic acid here is completely dissolved in 1.25 L of water. Then divide 0.1665 moles by 1.25 L to get the molar concentration, which will be 0.1332 M.
How do you determine the accurate concentration of an analyte? ›At the equivalence point the moles of added base will be equal to the moles of original acid, this allows the determination of the number of moles of original acid. This can then be combined with the original volume of the analyte solution to determine its concentration.
How can you determine the concentration of an unknown solution using a spectrophotometer? ›- Procedure:
- Turn on the spectrophotometer and allow it to warm up for 20 minutes.
- Blank the spec according to manufacturer's instructions using a wavelength of 520 nm.
- Set the mode to absorbance for data collection.
- Insert one known sample into the chamber.
- Record the absorbance value in the data table.
To do this measure called molarity is commonly used. Molarity (M) is defined as the number of moles of solute (n) divided by the volume (V) of the solution in liters. It is important to note that the molarity is defined as moles of solute per liter of solution, not moles of solute per liter of solvent.
What are the 2 ways we measure concentration? ›Concentration of a Solute. There are two basic ways of reporting the concentration of a solute in a solvent, by reporting the mass of solute in a given volume, or the number of moles of solute in a given volume.
What two factors determine the concentration of a solution? ›- Number of moles.
- Volume of the solution in liter.
- Concentration in parts per million.
- Mass Percentage.
- Volume Percentage.
- Mass by Volume Percentage.
- Molarity.
- Molality.
- Normality.
- Formality.
To concentrate a solution, one must add more solute, or reduce the amount of solvent (for instance, by selective evaporation). By contrast, to dilute a solution, one must add more solvent, or reduce the amount of solute. There exists a concentration at which no further solute will dissolve in a solution.
What is an example of concentration? ›The concentration of the solution tells you how much solute has been dissolved in the solvent. For example, if you add one teaspoon to two cups of water, the concentration could be reported as 1 t salt per 2 c water.
What is the most common measure of concentration? ›Molarity is the most common concentration unit. It is a measure of the number of moles of solute in one liter of solution. Molarity measurements are denoted by the capital letter M with units of moles/Liter. This shows the number of moles of solute dissolved in a liquid to make one liter of solution.