150
Stoichiometry I Calculations Based on Formulas
4 00) Do not worry about this Instead, realize that there will always be a little
experimental error that you will have to allow for Normally, look for relatively
simple numbers for subscripts
PROBLEM:
Many crystalline compounds contain water of crystallization that is driven off
when the compound is heated The loss of weight in heating can be used to determine the formula For example, a hydrate of barium chloride, BaCl 2 vH 2 O,
weighing 1 222 g is heated until all the combined water is expelled The dry powder remaining weighs 1 042 g Compute the formula for the hydrate
SOLUTION:
In solving this problem we seek the value of \ whose units must be moles of H 2 O
per mole of BaCl 2 Our objective, therefore, is to find the number of moles of each
in the given sample The 1 042 g residue is anhydrous BaCl 2 and the loss in weight
is H 2 O
Weight of H 2 O = 1 222 g - 1 042 g - 0 180 g H 2 O
From these weights we calculate the moles
Moles of BaCI 2 = ' °
42 B BaC '
2
= 0 OOSOO moles BaCl 2
2084
mole BaCl 2
Moles of H 2 O =
!
g
2
= 0 0100 moles H 2 O
180 mole H 2 0
We obtain v, the moles of H 2 O per mole of BaCl 2 , by dividing the moles of H 2 O by
the moles of BaCl 2 to which the water is attached to give
_ 00100 moles H 2 O _ moles H 2 O
0 OOSOO moles BaCl 2
mole BaCl 2
and the formula for the hydrate is BaCl 2 2H 2 O
DETERMINATION OF ATOMIC WEIGHTS
There is a simple way by which you can find the approximate atomic weight of
an element It is described on pp 211-212 With this method, you determine the
specific heat of the element experimentally, then divide it into 6 2 (using the rule
of Dulong and Petit), the result is the approximate atomic weight. Even if you
didn't know the name of the element, you could use this approximate value
along with an accurate chemical analysis to find dnaciuiate value of the atomic
weight
Stoichiometry I Calculations Based on Formulas
4 00) Do not worry about this Instead, realize that there will always be a little
experimental error that you will have to allow for Normally, look for relatively
simple numbers for subscripts
PROBLEM:
Many crystalline compounds contain water of crystallization that is driven off
when the compound is heated The loss of weight in heating can be used to determine the formula For example, a hydrate of barium chloride, BaCl 2 vH 2 O,
weighing 1 222 g is heated until all the combined water is expelled The dry powder remaining weighs 1 042 g Compute the formula for the hydrate
SOLUTION:
In solving this problem we seek the value of \ whose units must be moles of H 2 O
per mole of BaCl 2 Our objective, therefore, is to find the number of moles of each
in the given sample The 1 042 g residue is anhydrous BaCl 2 and the loss in weight
is H 2 O
Weight of H 2 O = 1 222 g - 1 042 g - 0 180 g H 2 O
From these weights we calculate the moles
Moles of BaCI 2 = ' °
42 B BaC '
2
= 0 OOSOO moles BaCl 2
2084
mole BaCl 2
Moles of H 2 O =
!
g
2
= 0 0100 moles H 2 O
180 mole H 2 0
We obtain v, the moles of H 2 O per mole of BaCl 2 , by dividing the moles of H 2 O by
the moles of BaCl 2 to which the water is attached to give
_ 00100 moles H 2 O _ moles H 2 O
0 OOSOO moles BaCl 2
mole BaCl 2
and the formula for the hydrate is BaCl 2 2H 2 O
DETERMINATION OF ATOMIC WEIGHTS
There is a simple way by which you can find the approximate atomic weight of
an element It is described on pp 211-212 With this method, you determine the
specific heat of the element experimentally, then divide it into 6 2 (using the rule
of Dulong and Petit), the result is the approximate atomic weight. Even if you
didn't know the name of the element, you could use this approximate value
along with an accurate chemical analysis to find dnaciuiate value of the atomic
weight