These are questions on the pre-lab of Synthesis and Analysis of Alum Crystals. 1
ID: 940717 • Letter: T
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?
Explanation / Answer
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
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