How do you find the percentage of water in a compound?

Answer 1

Follow these steps to determine the percent composition

• Write the molecular formula of the compound
• Determine the molar mass of the compound
• Divide the mass of the element in the compound by the molar mass

Determining the molar massAssuming you know the molecular formula, find a Periodic Table so you can determine the molar mass of the compound. You do this by adding together the atomic weights of the elements in the compound. Make sure to account for cases when there is more than one atom of an element.

For example, we'll calculate the molar mass of magnesium sulfate, MgSO4. From a periodic table, we know the atomic weights of the elements:

• Magnesium = 24.31 grams
• Sulfur = 32.06 grams
• Oxygen = 16.00 grams

The molar mass is just the sum of these numbers. But because there are four atoms of oxygen in the compound, you have to multiply 16.00 by 4.

Molar mass of MgSO4 = 24.31 + 32.06 + 16.00(4) = 120.37 grams/mole

Percent compositionNow that you know the molar mass, take the weight of the element you're determining and divide it by the molar mass. Multiply by 100 to get the percent.

Say that we're finding the percent of sulfur in magnesium sulfate:

32.06 ÷ 120.37 = .266 × 100 = 26.6% S in MgSO4

Or, the percent of oxygen in magnesium sulfate:

16.00(4) ÷ 120.37 = .532 × 100 = 53.2% O in MgSO4

A great resource for checking your answers is Wolfram|Alfa, which is in the related links section below. Check out the other links and questions for more information.

Answer 2

Example: H2O
Hydrogen's AMU is 1.01
Oxygen's AMU is 16.00
There are two hydrogens and one oxygen, so multiply 1.01 by 2 and multiply 16.00 by 1. You get 2.02 and 16.00. Add these together to get 18.02. This is called the formula mass. !6.00 of the 18.02 AMU is made up of oxygen, so divide 16.00 by 18.02. You get 0.8879. Multiply by 100. 88.79%Oxygen. 2.02 of the 18.02 AMU is made up of hydrogen, so divide 2.02 by 18.02. You get 0.1121. Multiply by 100. 11.21% Hydrogen.

Answer 3

First, calculate the molar mass of the compound. This is done by counting the number of each element and multiplying that number by the element's mass number, then adding up all of the resulting masses.

Then, go back to the mass for the element you want to know about, and divide this by the total masses, then multiply by 100. This is the percentage by mass.

Answer 4

(total mass of element present/molecular mass)*100

Answer 5

%composition=(element weight/compound weight) x100

Answer 6

A chemical analysis is necessary.

Answer 7

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