Question

These are questions on the pre-lab of Synthesis and Analysis of Alum Crystals.

1. 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. 2. 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

1) now

mass fo crucible = 24.330

mass of sample + crucible = 31.571

so

mass of sample = ( mass of sample + crucible) - mass of crucible

mass of sample = 31.571 - 24.330

mass of sample = 7.241

we know that

moles = mass / molar mass

so

moles of sample = 7.241 / 159.609

moles of sample = 0.045367

now

after heating

mass of sample + crucible = 29.002

so

the lost is nothing but water

so

mass of water = 31.571 - 29.002

mass of water = 2.569

so

moles of water = 2.569 / 18

moles of water = 0.142722

now

so

moles of water / moles of sample = 0.142722 / 0.045367

moles of water / moles of sample = 3

so

x = 3

so

formula is

salt . 3 H20

2)

% error = (experimental - theoretical ) x 100 / theoretical

so

% error = (6.913 - 7) x 100/ 7

% error = -1.24

the negative sign means that , the obtained is less than the theoretical value