# Question 43840

Mar 12, 2017

The compound will be 57.5% Na, 40.0% O and 2.5% H by mass.

#### Explanation:

Here's how to do a "percent by mass" problem:

First, unless some other quantity is stated, start by assuming you have one mole of the compound. In this case, that is 40.0 g of NaOH.

Next, read the formula to determine the number of moles of each type of atom in the compound. One mole of NaOH contains one mole of Na, one mole of O and one mole of H.

Look up the molar mass of each element:

Na = 23.0 g/mol

O = 16.0 g/mol

H = 1.0 g/mol

Now, divide each atomic mass by the molar mass of the compound, then multiply by 100 to change the resulting decimal into a percentage:

Na: (23g) / (40 g) * 100 = 57.5 %

O: (16g) / (40 g) * 100 = 40.0 %

H: (1.0g) / (40 g) * 100 = 2.5 %#