Question

7. A student conducts an experiment on a different hydrate. The empty crucible is heated to a constant
mass of 24.330. A sample of the unidentified hydrate is placed in the crucible, and the total mass is
31.571 g. The crucible and hydrate are heated to constant mass, which is measured at 29.002 g. The
molar mass of the anhydrous salt is 159.609 g.
a. Calculate the formula of this hydrate. Note that in the formula of a hydrate, salt•xH2O, the
coefficient “x” must be a whole number.
b. Give a reason your calculated value of “x” is less than the whole number to which you
rounded.

8. A student dehydrates a sample of magnesium sulfate heptahydrate. The anhydrous salt to water
ratio is 1:6.913. The correct formula is salt•7H2O.
a. Calculate the student’s percent error.
b. What is the physical meaning of the negative sign in the answer?

Answer 1

moles of salt=(29.002-24.330)/159.609

Moles of hydrate:(31.571-29.002)/18

now find the ratio of hydratemoles to salt moles:
x=moleshydrate/molessalt