Converting Grams of Substances to Moles and Grams with Solved Problems

Converting Moles of Substances to Grams Solved Problems

ZnI₂, can be prepared by the direct combination of elements. A chemist determines from the amounts of elements that 0.0654 mol ZnI₂ 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

H₂O₂ 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 H₂O₂ in 1.00 L of solution. What is the mass of H₂O₂ in this volume of solution?.

Boric acid, H₃BO₃ is a mild antiseptic and is often used as an eye wash. A sample contains 0.543 mol H₃BO₃. What is the mass of boric acid in the sample?.

CS₂ is a colourless, highly inflammable liquid used in the manufacture of rayon and cellophane. A sample contains 0.0205 mol CS₂. Calculate the mass of CS₂ 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, HNO₃ 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 HNO₃ was poured into a beaker. How many moles of HNO₃ are there in this sample of HNO₃?

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 N₂H₄

5.     7.05 g Br2

6.     35.4 g Li2 CO3

7.     7.83 g P4

_8.        153 g Al₂ (SO₄)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 10²³ 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 Br₂

 Number of atoms in 7.46 g Li