Calculate mass percentage

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To step
Method 1 of 3: Get started
Method 2 of 3: How do you determine the mass percentage for given masses
Method 3 of 3: How do you determine the mass percentage, if the masses are not given
Tips

You will be asked to determine the "mass percentage" of a particular chemical for a chemistry test. Before you panic, read on first. It's much easier than it sounds.

To step

Method 1 of 3: Get started

A percentage by mass is the mass of a particular chemical divided by the total mass of all chemicals in a solution / compound, expressed as a percentage. There are two ways to approach this mass percentage question. Read on below to determine which approach to take.

Determine whether the question you need to solve gives you the masses of the chemicals or not.
- When the masses are given. If the mass of the chemical substances is already given in the question, as with "What is the percentage by mass of 5 g of sodium hydroxide dissolved in 100 g of water?"then follow the instructions as described below in the section "How to determine the mass percentage for given masses".
- If the masses are not given. Sometimes you will be asked to provide the mass percentage, but the mass of the chemicals is unknown. In these cases you will use the chemical formulas to dissolve the chemical masses. Such a question might look like this, "Determine the mass percentage of hydrogen in a water molecule?" If this is the case, follow the instructions in the section "How to determine the mass percentage, if the masses are not given".

Method 2 of 3: How do you determine the mass percentage for given masses

Below you will find the steps required to solve a question about mass percentage where the mass of the chemical substances is given. Such a question might look like this, "What is the percentage by mass of 5 g of sodium hydroxide dissolved in 100 g of water?" The examples in this section elaborate on this question.

Calculate the total mass. Add up all the masses of all elements in the compound or solution. This will give you the total mass. This is the denominator. Write these down.
- For example, in this step you would do 100 g + 5 g for the total mass of 105 g.
Determine what the requested chemical is. When asked to find the "mass percentage", you are asked to determine the mass of a particular chemical (the chemical requested), as a percentage of the total mass of all elements. Determine what your chemical in question. Write these down. This is your counter.
- For example, in this step you determine that the requested chemical is 5 g of sodium hydroxide.
Share. Divide the mass of the requested chemical, found in the step "Determine the requested chemical", by the total mass as calculated in "Calculate the total mass". The result of this division is the mass.
- For example, in this step, you divide 5 g (mass of sodium hydroxide) by 105 (total mass) to get the mass ratio, 0.04761.
Calculate the percentage. Multiply the ratio of the mass found by 100. This gives the mass percentage.
- For example, in this step you multiply 0.04761 by 100 to get 4.761%. Thus, the mass percentage of 5 grams of sodium hydroxide dissolved in 100 grams of water is 4.761%.

Method 3 of 3: How do you determine the mass percentage, if the masses are not given

Below you will find the steps required to solve a question about mass percentages where the chemical masses are not given. Such a question might look like this, "Determine the mass percentage of hydrogen in a water molecule?The examples in this section elaborate on this problem.

Read the following definitions. Before you can calculate the mass percentage in such a statement, you will need to understand the chemical concepts below.
- Composite fabrics. A compound is formed by combining two or more different chemical elements. These elements are held together by covalent bonds or ionic bonds. The elements in composite materials can be separated again by chemical reaction.
  - For instance. Hydrogen and oxygen are different chemical elements. A water molecule is therefore a compound substance because it consists of two hydrogen atoms and one oxygen atom.
- Chemical formulas. A compound substance can be listed in an abbreviated form. This is called a chemical formula. A chemical formula also takes into account the relative amount of each atom in a compound.
  - For instance. The chemical formula of water consists of an "H" for hydrogen and an "O" for oxygen. Since there are two hydrogen atoms on every oxygen atom in a water molecule, the chemical formula of water should consist of two H's per O. Thus, the chemical formula of water is written as H2O.
- The molar ratio. The relative amount of one type of atom compared to that of another type of atom in a compound is called the number of moles. You can find this by looking at the chemical formula of a compound.
  - For instance. The chemical formula of water is H2O. By decomposing this formula, we know that the molar ratio of hydrogen and oxygen in a water molecule is equal to 2: 1.
Write out the chemical formula. Perhaps the chemical formulas have been given, but if not, first write down the chemical formulas of each requested compound. If the chemical formulas are given, you can skip this step and continue with the step "Determine the mass of each element".
- For example, in this step you have to write the chemical formula of water as H2O.
Determines the mass of each element. Look up the molecular weight of each element in the chemical formula on the periodic table. Write these down.
- For example, in this step you are looking for the molecular weight of oxygen, 15.9994; and the molecular weight of hydrogen, 1.00794.
Multiply the masses by the molar ratio. Determine how many moles are in each element of the compound. Multiply the molar mass by the molar ratio. Write these down.
- For example, in this step, the molar ratio of hydrogen and oxygen in water is 2: 1.Therefore, you multiply the molar mass of hydrogen by 2.100794 X 2 = 2.01588; and leave the molar mass of oxygen as it is, 15.9994.
Calculate the total mass. Add the total mass of all elements in your compound. Make sure to use the masses from the "Multiply the masses by the mole ratio" step to account for the correct mole ratios. Write these down. This will be your denominator.
- For example, in this step, you add 2.01588 (the mass of 2 moles of hydrogen atoms) by 15.9994 (the mass of 1 mole of oxygen atoms) and you get 18.01528.
Determine the requested chemical. When you are asked to calculate the "percentage by mass", you are asked to determine the mass of a particular chemical (the chemical requested) as a percentage of the total mass of all elements. Determine the requested chemical. Write these down. This is the denominator.
- For example, in this step you discover that the chemical requested is hydrogen.
Share. Divide the mass of the requested chemical from the step "Determine the requested chemical" by the total mass calculated in "Calculate the total mass". This calculated number is the mass ratio.
- For example, in this step you divide 2.01588 (the mass of hydrogen atoms) by 18.01528 (total mass of a water molecule). This gives you a mass ratio of 0.11189.
Calculate the percentage. Multiply the resulting mass ratio ratio from the "Divide" step by 100. This will give you the answer, the mass percentage.
- For example, in this step you multiply 0.11189 by 100 to get 11.18%. The mass percentage of hydrogen atoms in a water molecule is 11.18%.

Tips

Make sure your chemical formula is balanced in the "Write the chemical formula" step. If the chemical formula is given, it should be in balance. However, if you were first asked to solve a chemical equation to create the aforementioned chemical formula, make sure it is balanced before determining the mass percentage.