Question

What is the percentage of water in the following compound? Answer using three significant figures. Sodium carbonate decahydrate, Na2CO3 • 10H2O % by mass H2O

Answer 1

To find the percentage of water in the compound Sodium carbonate decahydrate, divide the molar mass of water by the molar mass of the whole compound and multiply by 100%. In this case, the percentage of water is 62.937%.

Step-by-step explanation:

To find the percentage of water in the compound Sodium carbonate decahydrate (Na2CO3 • 10H2O), we need to calculate the mass of water and the mass of the whole compound. The molar mass of water is 18g/mol. The molar mass of the compound can be calculated by adding the molar masses of all the elements present in it.

The molar mass of Na2CO3 is 106 g/mol (22.99 g/mol Na x 2 + 12.01 g/mol C + 16.00 g/mol O x 3) and the molar mass of 10H2O is 180 g/mol (18.00 g/mol H2O x 10). So the molar mass of the whole compound is 286 g/mol (106 g/mol + 180 g/mol).

To calculate the percentage of water, divide the molar mass of water by the molar mass of the whole compound and multiply by 100%. In this case, the percentage of water in Sodium carbonate decahydrate is 62.937%.

Answer 2

The trick for this problem is to understand atomic mass: the fact that different atoms have different masses. What we need to do is add up all the atomic masses of the compound and work out the ratio of mass of water to the mass of sodium carbonate. Atomic masses are often given for each atom in the periodic table, but you can look them up on google too.

You can do this by adding up individual atoms for each molecule, or you can shortcut and lookup the molar mass of the compound (i.e.the task already done for you).

The molar mass of water is 18.01g/mole so for 10 moles of water we have a mass of 180.1g.


The molar mass of sodium carbonate is 106g/mole (google).

So the total mass of the sodium carbonate decahydrate compound is 180.1+106 = 286.1g, of which water would make up 180.1g, so the percentage of water is is 180.1/286.1 = 0.629, so we can round this to 63%

:)