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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Converting Grams of Substances to Moles and Grams with 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.

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

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