What is the formula mass of Na2CO3.10H2O?

This is not difficult if you remember the meaning of numbers in different positions. The correct format is Na2CO3.10H2O (sodium carbonate decahydrate). The subscripts multiply only the element immediately before them, and the 10 multiplies the whole water molecule. Thus we have:sodium, Na, 2 moles of atoms

carbon, C, 1 mole of atoms

oxygen, O 3 moles of atoms

water, H2O, 10 moles of molecules, i.e. 20 moles of hydrogen atoms, H, and 10 moles of oxygen atoms, O.

Collecting these we have 2 moles Na, I mole C, 13 moles O and 20 moles of H.

We find the formula mass by multiplying each element's number of moles by its mole mass and adding them all up:

r.f.m = 2 x 23 + 12 +13 x 16 + 20

= 286.