Updated August 07, 2017
By Ryan Menezes
Density and concentration both describe the amount of a solute per unit volume of a solvent. The former value measures mass per volume. The latter value measures how many moles of atoms exist per unit volume. The solute's mass tells you how many moles it contains. You can calculate the solution's mass as long as you know the solute and solvent's mass. The solution's density then lets you calculate its volume.
Divide the solute's mass by its molar mass. If, for instance, the solution contains 30 grams of silver nitrate, which has a molar mass of 169.88: 30 / 169.88 = 0.176 moles.
Add the mass of the solute to the mass of the solvent. If the silver nitrate is dissolved in 70 grams of water: 30 + 70 = 100 grams.
Divide this answer by the solution's density. If its density is 1.622 grams per cubic centimeter: 100 / 1.622 = 61.65. This answer is the solution's volume, measured in cubic centimeters.
Divide your answer by 1,000 to convert it to liters: 61.65 / 1,000 = 0.06165.
Divide the answer to Step 1 by the answer to Step 4: 0.176 / 0.06165 = 2.85 moles per liter.
Updated April 26, 2018
By Oxana Fox
Concentration represents the amount of the compound dissolved in the solution. Molarity is the number of moles of a substance in 1 liter of the solution. Another unit of the concentration, weight percentage, refers to the ratio of the mass of the solute (a dissolved substance) to the mass of the solution. Converting between concentrations is frequently required for various problems in chemistry.
Determine atomic masses of elements that comprise the dissolved compound using the Periodic Table of the Elements. For example, if the compound in the solution is potassium chloride (KCl), the atomic mass of potassium (K) is 39 and that of chlorine (Cl) is 35.5.
Multiply the atomic mass by the number of the respective atoms in the molecule, and then sum up the products to calculate the molar mass In this example, the molar mass of KCl is 39 x 1 + 35.5 x 1 = 74.5.
Multiply the molar mass of the compound by the molarity to calculate the amount of the dissolved substance in one liter of the solution. For example, 0.5 M of KCl solution contains 74.5 x 0.5 = 37.25 g of the salt.
Multiply the density of the solution by 1,000 ml (1 liter) to calculate the mass of the 1L of the solution. For example, if the density of 0.5 M KCl solution is 1.1 g/ml, the weight of 1 liter of the solution is 1.1 x 1,000 = 1,100 g.
Divide the mass of the dissolved compound by the mass of the solution, and multiply the result by 100 to calculate percentage. In this example, the solution of KCl is (37.25 ÷ 1,100) x 100 = 3.39 percent.
Mass percentage is one way of representing the concentration of an element in a compound or a component in a mixture. Mass percentage is calculated as the mass of a component divided by the total mass of the mixture, multiplied by 100%.
Also Known As: mass percent, (w/w)%
Mass percent is the mass of the element or solute divided by the mass of the compound or solute. The result is multiplied by 100 to give a percent.
The formula for the amount of an element in a compound is:
mass percent = (mass of element in 1 mole of compound / mass of 1 mole of compound) x 100
The formula for a solution is:
mass percent = (grams of solute / grams of solute plus solvent) x 100
or
mass percent = (grams of solute / grams of solution) x 100
The final answer is given as %.
Example 1: Ordinary bleach is 5.25% NaOCl by mass, which means each 100 g of bleach contains 5.25 g NaOCl.
Example 2: Find the mass percentage of 6 g sodium hydroxide dissolved in 50 g of water. (Note: since the density of water is nearly 1, this type of question often gives the volume of water in milliliters.)
First find the total mass of the solution:
total mass = 6 g sodium hydroxide + 50 g water
total mass = 56 g
Now, you can find the mass percentage of the sodium hydroxide using the formula:
mass percent = (grams of solute / grams of solution) x 100mass percent = (6 g NaOH / 56 g solution) x 100mass percent = (0.1074) x 100
answer = 10.74% NaOH
Example 3: Find the masses of sodium chloride and water required to obtain 175 g of a 15% solution.
This problem is a bit different because it gives you the mass percentage and asks you to then find how much solute and solvent are needed to yield a total mass of 175 grams. Start with the usual equation and fill in the given information:
mass percent = (grams solute / grams solution) x 100
15% = (x grams sodium chloride / 175 g total) x 100
Solving for x will give you the amount of NaCl:
x = 15 x 175 / 100
x = 26.25 grams NaCl
So, now you know how much salt is needed. The solution consists of the sum of the amount of salt and water. Simply subtract the mass of salt from the solution to obtain the mass of water that is required:
mass of water = total mass - mass of saltmass of water = 175 g - 26.25 g
mass of water = 147.75 g
Example 4: What is the mass percent of hydrogen in water?
First, you need the formula for water, which is H2O. Next you look up the mass for 1 mole of hydrogen and oxygen (the atomic masses) using a periodic table.
hydrogen mass = 1.008 grams per mole
oxygen mass = 16.00 grams per mole
Next, you use the mass percentage formula. The key to performing the calculation correctly is to note there are 2 atoms of hydrogen in each water molecule. So, in 1 mole of water there are 2 x 1.008 grams of hydrogen. The total mass of the compound is the sum of the mass of the two hydrogen atoms and one oxygen atom.
mass percent = (mass of element in 1 mole of compound / mass of 1 mole of compound) x 100mass percent hydrogen = [(2 x 1.008) / (2 x 1.008 + 16.00)] x 100mass percent hydrogen = (2.016 / 18.016) x 100
mass percentage hydrogen = 11.19%