13. A quantity of epsom salts, magnesium sulfate heptahydrate, MgSO4*7H2O..?

Question

...is heated until all the water is driven off. The sample

loses 11.8 g in the process. What was the mass of the original

sample?

14. The process of manufacturing sulfuric acid begins with the burning

of sulfur. What mass of sulfur would have to be burned in order to

produce 1.00 kg of H2SO4 ? Assume that all of the sulfur ends up in

the sulfuric acid.

Answer 1

Molecular mass of H2O = (2 x 1.008) + 16.00 = 18.016 g/mol

Molecular mass od MgSO4.7H2O = 24.31 + 32.07 + (11 x 16.00) + (14 x 1.008) = 246.492 g/mol

The MgSO4 is combined with the H2O in the ratio 1:7. This means that for every 1 mole of MgSO4 there are 7 moles of H2O.

Since you have removed all of the H2O from the sample by heating then the number of moles of H2O you have removed will be 7 x the moles of  MgSO4.7H2O that you have left

MgSO4.7H2O ----------------> MgSO4 +  7H2O

Moles = mass / molecular weight

moles H2O = 11.8 g / 18.016 g/mol

= 0.655 moles of water was in the MgSO4.7H2O

Therefore moles of original sample = 0.655 / 7

= 0.0936 moles of MgSO4.7H2O

mass MgSO4.7H2O = moles x molecular weight

= 0.0936 mol x 246.492 g/mol

= 23.06 g

14.

molecular mass H2SO4 = (2 x 1.008) + 32.07 + (4 x 16.00) = 98.086 g/mol

moles = mass / molecular weight

moles of H2SO4 in 1 kg = 1000 g / 98.086 g/mol

= 10.2 moles of H2SO4

There is 1 S atom in every molecule of H2SO4, therefore in 1 mole of H2SO4 there is 1 mole of S

So in 10.2 moles there are 10.2 moles of S

molecelas mass of S = 32.07 g/mol

mass S = moles x molecular mass

= 10.2 mol x 32.07 g/mol

= 327 g