1. calculate percentage of:
a)NaOH
b)C6 H6
c)CH4
d)CH3 CH3 OH
e)Mg (oH)2
Answers
Answer:
Write down the formulae of
(i) Sodium oxide
(ii) Aluminium chloride
(iii) Sodium sulphide
(iv) Magnesium hydroxide
Ans. The formulae. are
(i) Formula bf Sodium Oxide
Symbol → Na O
Charge → +1 –2
Formula → Na2O
(iii) Formula of Sodium Oxide
Symbol → Na S
Charge → + 1 –2
Formula → Na2S
(ii) Formula of aluminium chloride
Symbol → Al Cl
Charge → +3 –1
Formula → A1C13
(iv) Formula of magnesium hydroxide
Symbol → Mg OH
Charge 1
Formula → Mg(OH)2
2. Write down the names of compounds represented btthe following formulae:
(i) Al2(SO4)3 (ii) CaCl2 (iii) K2SO4 (iv) KNO3
(v) CaCO3
Ans. (i) A12(SO4)3 → Aluminium sulphate
(ii) CaC12 → Calcium chloride
(iii) K∴SO4 → Potassium sulphate
(iv) KNO3 → Potassium nitrate
(v) CaCO3 → Calcium carbonate
3. What is meant by the term chemical formula?
Ans. The chemical formula of the compound is a symbolic representation of its composition, e.g., chemical formula of sodium chloride is NaCl.
4. How many atoms are present in a
(i) H2S molecule and
(ii) PO43– ion?
Ans. (i) H2S → 3 atoms are present
(ii) PO43– → 5 atoms are present
NCERT TEXTBOOK PAGE 40
1. Calculate the molecular masses of H2, O2, C12, CO2, CH4, C2H6, C2H4, NH3, CH3OH.
Ans. The molecular masses are:
H2 ⇒ 1 × 2 → 2u
O2⇒ 16 × 2 → 32u
Cl2 ⇒ 35.5 × 2 → 71 u
CO2 ⇒ 1 × 12 + 2 × 16 = 12 +32 = 44 u
CH4 ⇒ 1 × 12 + 4 × 1 = 16 u
C2H6 ⇒�n2 × 12 + 6 × 1 = 30 u
C2H4 ⇒ (2 × 12) + (4 × 1) = 28 u
NH3 ⇒ (1 × 14) + (3 × 1) = 17 u
CH3OH ⇒ 12 + (3 × 1) + 16 + 1 = 32 u
2. Calculate the formula unit masses of ZnO, Na2O, K2Co3 given atomic masses of Zn=65 u, Na = 23 u, K = 39 u, C = 12u, and O = 16 u.
Ans. The formula unit mass of
(i) ZnO = 65 u + 16 u =.81 u
(ii) Na2O = (23 u × 2) + .16 u : = 46u+ 161.u.
= 62 u
(iii) K2CO3 = (39 u × 2) + 12 u + 16 u × 3
= 75 u + 12u = 48u = 138u