In chemistry, the concentration of a solution is the amount of a substance that is dissolved, called the solute, which is mixed with another substance, called the solvent. The standard formula is C = m/V, where C is the concentration, m is the mass of the solute, and V is the total volume of the solution. If your solution has a small concentration, look up the answer in parts per million (bpd) to make it easier to understand. While in the laboratory, you may be asked to find the molarity or molar concentration of the related solution.
Step
Method 1 of 3: Using the Mass per Volume Formula
Step 1. Find the mass of the solute mixed with the solvent
A solute is a substance that is mixed to form a solution. If the problem gives the mass value of the solute, write it down and make sure you put the correct unit. If you need to find the mass of a solute, measure it on a scale and record the result.
If the solute used is a liquid, you can also calculate the mass using the density formula: D = m/V, where D is the density, m is the mass of the liquid, and V is the volume. To find the mass, multiply the density of the liquid by the volume
Tip:
If you need to use a scale, reduce the mass of the container used to hold the solute to get an accurate result.
Step 2. Record the total volume of the solution
The total volume of a solution is the amount of solvent plus the amount of solute mixed. If you are looking for volume in a laboratory, mix the solution in a cylinder or measuring beaker and see what size it is. Measure the volume from the indentation at the top of the solution (the meniscus) for the most accurate measurement. Record the volume of the solution obtained.
- If you're not measuring volume yourself, you may need to convert the mass of the solute to volume using the density formula.
- For example, if you want to find the concentration of 3.45 grams of salt in 2 liters of water, find the volume using the density formula. Look up the density of salt in a textbook or online, and use it to find the value of m. In this case, the density of the salt is 2.16 g/ml. Thus, the formula becomes 2.16 g/ml = (3.45 g)/V. Multiply each side by V to get V(2.16 g/ml) = 3.45 g. Then, divide each side of the equation by 2.16 to find the volume value, which is V = (3.45 g)/(2.16 g/ml) = 1.60 ml.
- Add the volume of solute to the volume of solvent. So, in this example, 2 L + 1.6 ml = 2,000 ml + 1.6 ml = 2.001.6 ml. You can either leave the units in milliliters (ml) or change them back to liters and get 2.002 L.
Step 3. Divide the mass of the solute by the total volume of the solution
Use the formula C = m/V, where m is the mass of the solute and V is the total volume of the solution. Enter the previously searched mass and volume values, then divide to find the solution concentration value. Don't forget to put the correct units.
- In this example, for a concentration of 3.45 grams of salt in 2 liters of water, the equation is C = (3.45 g)/(2.002 L) = 1.723 g/L.
- Sometimes, questions ask for answers in certain units. Make sure to convert the values to the correct units before plugging them into the final formula.
Method 2 of 3: Finding Concentration in Percentages or Parts per Million
Step 1. Find the mass of the solute in grams
Measure the mass of the solute you plan to mix in the solution. Make sure you subtract it from the mass of the container so that the concentration calculation is accurate.
If the solute is a liquid, you need to calculate the mass using the formula D = m/V, where D is the density of the liquid, m is the mass, and V is the volume. Look up the density of the liquid in a textbook or online to solve the above formula
Step 2. Determine the total mass of the solution in grams
The total mass of the solution is the mass of the solvent plus the mass of the solute. Find the mass of a substance using a laboratory balance or convert the volume of solute to mass using the formula for density D = m/V. Add the mass of the solute to the mass of the solute to get the final volume.
For example, if you want to find the concentration of 10 grams of cocoa powder with 1.2 L of water, first find the mass of water using the density formula. The density of water is 1000 g/L so your formula will be 1000 g/L = m/(1, 2 L). Multiply each side by 1.2 L to get the mass in grams so that m = (1, 2 L)(1,000 g/L) = 1,200 grams. Add to the mass of cocoa powder to get 1,210 grams
Step 3. Divide the mass of the solute by the total mass of the solution
Write an equation so that concentration C = mass of solute/total mass of solution. Enter the values and solve the equation to find the concentration of the solution.
In our example, C = (10 g)/(1.210 g) = 0.00826
Step 4. Multiply the answer by 100 to find the concentration in percent
If you are asked to present your concentration as a percentage, multiply your answer by 100. Put a percentage symbol at the end of your answer.
In this example, the percent concentration is (0.00826)(100) = 0.826%
Step 5. Multiply the concentration by 1,000,000 to find the parts per million
Multiply the obtained concentration value and multiply by 1,000,000 or 106. The result is the number of parts per million (bpj) of solute. Put the bpj unit in the final answer.
In this example, bpj = (0, 00826)(1,000,000) = 8,260 bpd
Tip:
Parts per million are usually used for very small concentrations because they are easier to write and understand than percentages.
Method 3 of 3: Calculating Molarity
Step 1. Add the atomic masses of the solutes together to find the molar mass
Look at the element in the chemical formula for the solute used. List the atomic masses for each element in the solute because the atomic and molar masses are the same. Add up the atomic masses of the solutes to find the total molar mass. Label the final answer in g/mol.
- For example, if the solute is potassium hydroxide (KOH), find the atomic masses for potassium, oxygen, and hydrogen and add them all together. In this case the molar mass = 39 +16 + 1 = 56 g/mol.
- Molarity is primarily used in chemistry when you know the constituent elements of the solute used.
Step 2. Divide the mass of the solute by the molar mass to find the mole value
Find the mass of solute added to your solution using a lab balance, if needed. Make sure you reduce the mass of the container so that you get an accurate result. Divide the obtained mass by the molar mass to get the number of moles of solute used. Give the unit “mole” in the answer.
- For example, if you wanted to find the number of moles in 25 grams of potassium hydroxide (KOH), the equation would be moles = (25 g)/(56 g/mol) = 0.45 mol
- Convert the mass of the solute to grams if it is still in other units.
- The mole is used to represent the atomic number in a solution.
Step 3. Convert the volume of the solution to liters
Find the volume of solvent before you mix the solute. Use a flask or measuring cylinder to measure the volume of solvent if the value is not known. If the unit used is milliliters, divide by 1,000 to convert it to liters.
- In this example, if you are using 400 ml of water, divide by 1000 to convert it to liters, which is 0.4 L.
- If the solvent already has liters, just skip this step.
Tip:
You don't need to include the volume of the solute as it usually doesn't affect the volume much. If there is a significant change in volume when the solvent is mixed with the solute, use the total volume.
Step 4. Divide the moles of solute by the volume of the solution in liters
Write the equation for molarity M = mol/V, where mole is the number of moles in the solute and V is the volume of the solvent. Solve the equation and affix the unit M to the answer.
