Converting Moles of Substances to Grams Solved
Problems
ZnI_{2}, can be prepared by the direct combination of elements.
A chemist determines from the amounts of elements that 0.0654 mol ZnI_{2}
can be formed.
Solution
The molar mass of ZnI2 is 319 g/mol. (The formula weight is 319 amu,
which is obtained by summing the atomic weight in the formula) Thus
0.0654 mol ZnI2 x (319 g ZnI2/
1 mol ZnI2 ) = 20.9 gm ZnI2
Problems for Practice
H_{2}O_{2} is a colourless liquid. A concentrated
solution of it is used as a source of oxygen for Rocket propellant fuels.
Dilute aqueous solutions are used as a bleach. Analysis of a solution shows
that it contains 0.909 mol H_{2}O_{2} in 1.00 L of solution.
What is the mass of H_{2}O_{2} in this volume of solution?.
Boric acid, H_{3}BO_{3} is a mild antiseptic and is
often used as an eye wash. A sample contains 0.543 mol H_{3}BO_{3}.
What is the mass of boric acid in the sample?.
CS_{2} is a colourless, highly inflammable liquid used in the
manufacture of rayon and cellophane. A sample contains 0.0205 mol CS_{2}.
Calculate the mass of CS_{2} in the sample.
Converting Grams of Substances to Moles
In the preparation of lead(II)chromate PbCrO4,
45.6 g of lead(II)chromate is obtained as a precipitate. How many moles of
PbCrO4 is this?
The molar mass of PbCrO4 is 323 g/mol (i.e) 1
mol PbCrO4 = 323 g PbCrO4
Therefore,
45.6 g PbCrO4 x (1 mol.PbCrO4 / 323 g PbCrO4) = 0.141 mol PbCrO4
Problems for Practice
Nitric acid, HNO_{3} is a colourless, corrosive liquid used in
the manufacture of Nitrogen fertilizers and explosives. In an experiment to
develop new explosives for mining operations, a 28.5 g sample of HNO_{3}
was poured into a beaker. How many moles of HNO_{3} are there in this
sample of HNO_{3}?
Obtain the moles of substances in the following.
1.
3.43 g of C
2.
76g C4 H10
3.
e. 2.57 g As g.
_{4. }41.4 g N_{2}H_{4}
5. 7.05 g Br2
6.
35.4 g Li2 CO3
7. 7.83 g P4
_{8. }153 g Al_{2} (SO_{4})_{3}
Calculation of the
Number of Molecules in a Given Mass Solved Problem
How many molecules are there in a 3.46 g sample
of hydrogen chloride, HCl?
Note: The number of molecules in a sample is related to moles of
compound (1 mol HCl = 6.023 x 10^{23} HCl molecules). Therefore if you
first convert grams HCl to moles, then you can convert moles to number of
molecules).
Solution_{}
3.46g HClx ( 1molHCl/36.5g HCl ) x (
6.023x1023 HClmolecules / 1molHCl) = 5.71 x 1022 HCl molecules
Problems for Practice
How many molecules are there in 56mg HCN?
Calculate the following
Number of molecules in 43g NH3
Number of atoms in 32.0 g Br_{2}
Number of atoms in 7.46 g Li