These are questions on the pre-lab of Synthesis and Analysis of Alum Crystals. 1

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?

Please help with any step by step explainations completley explained out - thank you

Explanation / Answer

1)

mass of hydrate = total mass - mass of empty crucible

mass of hydrate = 31.571 - 24.33

mass of hydrate = 7.241

now

mass of water in hydrate = total mass ( crucible + hydrate) before heating - total mass after heating

mass of water in hydrate = 31.571 - 29.002

mass of water in hydrate = 2.569

now

mass of anhydrous salt = mass of hydrate - mass of water

mass of anhydrous salt = 7.241 - 2.569 = 4.672

now

moles = mass / molar mass

so

moles of anhydrous salt = 4.672 / 159.609 = 0.02927

moles of water = 2.569 / 18 = 0.1427222

now

moles ratio = 0.02927 : 0.142722 = 4.875

so

the formula of the hydrate is salt.5H20

2)

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

% error = -1.243 %

so

the percent erroor is -1.243

b)

the negative sign indicates that the actual value obtained is less than the theoretical value

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