The basic formula for mass percent of a compound is: mass percent = (mass of chemical/total mass of compound) x 100. You must multiply by 100 at the end to express the value as a percentage.  Write the equation at the beginning of every problem: mass percent = (mass of chemical/total mass of compound) x 100. Both of the values should be in grams so that they cancel each other out once you solve the equation. The mass of the chemical you’re interested in is the mass given in the problem. If the mass isn’t given, refer to the following section about solving for mass percent when the mass is not given. The total mass of the compound is calculated by summing the masses of all of the chemicals used to make the compound or solution. When you know the masses of all the elements or compounds being added together, you simply need to add them together to calculate the total mass of the final compound or solution. This will be the denominator in the mass percent calculation.  Example 1: What is the percent mass of 5g of sodium hydroxide dissolved in 100g of water? The total mass of the compound is the amount of sodium hydroxide plus the amount of water: 100g + 5g for a total mass of 105g.  Example 2: What masses of sodium chloride and water are needed to make 175 g of a 15% solution? In this example, you are given the total mass and the percentage you want, but are asked to find the amount of solute to add to the solution. The total mass is 175 g. When asked to find the "mass percent", you are being asked to find the mass of a particular chemical (the chemical-in-question) as a percentage of the total mass of all elements. Write down the mass of chemical-in-question. This mass will be the numerator in the mass percent calculation.  Example 1: The mass of the chemical-in-question is 5g of sodium hydroxide. Example 2: For this example, the mass of the chemical-in-question is the unknown you are trying to calculate. Once you have determined the values for each variable, plug them into the equation.  Example 1: mass percent = (mass of chemical/total mass of compound) x 100 = (5 g/105 g) x 100. Example 2: We want to rearrange the mass percent equation to solve for the unknown mass of the chemical: mass of the chemical = (mass percent*total mass of the compound)/100 = (15*175)/100. Now that the equation is filled in, simply solve to calculate the mass percent.  Divide the mass of the chemical by the total mass of the compound and multiply by 100. This will give you the mass percent of the chemical.  Example 1: (5/105) x 100 = 0.04761 x 100 = 4.761%. Thus, the mass percent of 5g of sodium hydroxide dissolved in 100g of water is 4.761%. Example 2: The rearranged equation to solve for mass of the chemical is (mass percent*total mass of the compound)/100: (15*175)/100 = (2625)/100 = 26.25 grams sodium chloride. The amount of water to be added is simply the total mass minus the mass of the chemical: 175 – 26.25 = 148.75 grams water.
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