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Determination of Formula through Experiment

Two basic ways by which the amount of each element in a given mass of compound is found is by synthesis or analysis.

• The process of synthesis is the formation of a compound from its elements or simpler compounds.

• Analysis involves the breaking down of a compound into its elements or smaller compounds.

The knowledge of the mass of each element in a known mass of. the compound is one of the keys to the molecular formula determination.

 

EVALUATE YOURSELF

 

  1. I.                  Objective Type Questions——–

Select the most appropriate choice from the options given as

(a),( b),( c) and (d) after each question:

1. Which of the following compounds has same empirical formula as that of glucose?

(a) CH2CHO                           (b) CH3COOH

(c) CH3OH                              (d) C2H6

 

2. Two elements X (atomic mass = 50) and Y (atomic mass = 16) combine to give a compound having 32% Y. The formula of the compound is

(a) H2Y3                                  (b) XY

(c) XY2                                    (d) X3Y4.

3. The vapour density of certain gas X is 4 times that of Y. The molecular mass of X is M. The molecular mass of Y is

(a) 4M                                     (b) 0.5 M

(c) 0.25 M                                (d) 0.33 M.

4. A symbol not only represents the name of the element but also represents

(a) its atomic number              (b) one gram-atom

(c) its atomicity                       (d) reactivity.

 

5. The balancing of equations is based upon which of the following law?

(a) Law of multiple proportions

(b) Law of conservation of mass

(c) Boyle’s law

(d) Law of reciprocal proportions.

6. Hydrogen phosphate of certain metal has a formula MHP04, the formula of metal chloride is

(a) MCI                                   (b) MCI3

(c) MCl2                                  (d) MCI4 .

 

7. An organic comp(>und containing C, H, N gave the following analysis:

C = 40%, H = 13.33%, N = 46.4 7%. Its empirical formula is

(a) C2H2N2                              (b) CH5N

(c) CH4N                                 (d) C2H7N

 

8. In the reaction, 4NH3 + 502 ~ 4NO + 6H20. When one mole of ammonia and one mole of oxygen are made to react to completion, then

(a) 1.0 mole of H2O is produced

(b) All the oxygen is consumed

(c) 1.0 mol of NO is formed

(d) All the ammonia is consumed.

9 100 g CaC03 is treated with 1 litre of 1 N HCI. What would be the weight of C02 liberated after the completion of the reaction?

(a) 5.5 g                       (b) 11 g

(c) 22 g                        (d) 33 g

(e) 44 g.

 

II. Fill in the Blanks———–

10. Complete the following sentences by supplying appropriate words:

(i) Hydrogen and oxygen are present in water in the ratio ….. . by mass.

(ii) A chemical compound always contains same elements combined together in same proportion by mass. This law is known as …… .

(iii) The number of moles of Kl required to produce 0.4 moles of K2Hgl4 by its reaction with HgCI4 in aqueous solution is ….. .

 

III. Discussion Questions ———-

 

11 What do you understand by chemical formula of a substance?

12. What is an ion? Give formulae of the following ions:

(i) Nitrate                    (ii) Nitride

(iii) Phosphate             (iv) Ammonium

(v) Manganese.

 

13. Write the formulae and names of the compounds formed between:

(i) Potassium and iodide ions

(ii) Sodium and sulphide ions

(iii) Aluminium and chloride ions.

 

14. Write the formulae of the following compounds:

(i) Sodium sulphate (ii) Ammonium phosphate

(iii) Calcium hydride (iv) Lithium nitride.

 

15. Write the formulae of the compounds formed by:

(i) Cr3+ and p- (it) Hg2+ and S2-

(iii) Pb2+ and PO/-.

16. What is a chemical equation? What is a balanced chemical equation? Why should the chemical equations be balanced?

17. 80 g of H2  are reacted with 80 g of 0 2 to form water. Find out the mass of water obtained. Which substance is the limiting reagent?

18. A chemical compound is found to have the following composition: K = 42.5%; Fe = 15.2%; C = 19.5% and N = 22.8%. Calculate the simplest formula of the compound.

19. An organic compound on analysis is found to have the following composition: C = 41.4%; H = 5.75%; N = 16.08% and rest is oxygen. Find out the empirical formula of the compound.

20. 1.615 g of an anhydrous salt was placed in moist air. Mter a few days it was found to be H2875 g. Assuming that complete hydration has taken place, calculate the simplest formula of hydrated salt. The composition of anhydrous salt is Zn = 40.6%; S = 19.8%; 0 = 39.6%.

21. What is the empirical formula of the mineral having the composition CaO = 48%, Pp5 = 41.3%; CaCH2 = 10.7%?

22. The percentage composition of Mohr’s salt is as follows: Fe2+= 14.32%; NH/= 9.20%, SO2-4-= 49%;Rest being water of crystalisation. Find the empirical formula of Mohr’s salt.

23. An organic compound has the following percentage composition C = 12.36%; H = 2.13%; Br = 85%. Its V.D. is 94. Find its molecular formula.

24. A compound contains 32% carbon, 4% hydrogen and rest oxygen. Its vapour density is 75. Calculate the empirical and molecular formula.

25 A crystalline salt on being rendered anhydrous loses 46% of its mass. The percentage composition of anhydrous salt is: Al = 10.50%, K = 15.1 %, S = 24.8%, 0 = 49.6%. Find the simplest formula of the anhydrous and crystalline salt.

26. Calculate the empirical formula of a compound whose percentage composition is: C = 21.9%, H = 4.6%, Br = 73.4%.

27 An acid of molecular mass 104 contains 34.6% carbon, 3.85% hydrogen and the rest is oxygen. Calculate the molecular formula of the acid.

28. State and explain the law of multiple proportions.

 

29. State and explain law of definite proportions.

30. How is empirical formula of a compound related to its molecular formula?

31. What are the various steps in calculating the molecular formula of a compound from the percentage composition of the compound?

32. Can two different compounds have same empirical formula? illustrate your answer with two examples.

33. Can two different compounds have same molecular formula? illustrate your answer with two examples.

34 What do you understand by the terms? (t) Limiting reagent (ii) Stoichiometry

35. Calculate the amount of carbon dioxide that could be produced when

(i) 1 mole of carbon is burnt in air

(ii) 1 mole of carbon is burnt in 16 g of dioxygen.

36. Determine the molecular formula of an oxide of iron in which mass percent of iron and oxygen are 69.9 and 30.1 respectively. Given molar mass of oxide is 159.8 g mol-1 (at. mass:Fe= 55.85, 0 = 16.00 u).