Question 78c82

Apr 16, 2015

First you have to find molar mass for water in general.

Mr (${H}_{2} O$) = Ar (O) + 2 $\cdot$ Ar (H)

Mr (${H}_{2} O$) = 15.999 + 2 $\cdot$ 1.007

Mr (${H}_{2} O$) = 18.013

M (${H}_{2} O$) = 18.013 g/mole

When you have 2 molecules ---> 2 $\cdot$ 18.013 g/mole = 36.026 g/mole

Apr 16, 2015

There is no such thing as a molar mass of 2H₂O.

The molar mass is by definition the mass of one mole.

The mass of 1 mol of H₂O is (2×1.008 + 1×15.999) g = 18.015 g.

So the molar mass of water is 18.015 g/mol.

Of course, the mass of 2 mol of water is

2 cancel("mol") × "18.015 g"/(1 cancel("mol")) = "36.030 g"#.

But that's not the molar mass of 2H₂O.