Question 19: How many significant figures are present in the following?

- 0.0025
- 208
- 5005
- 126,000
- 500.0
- 2.0034

**Answer:** (i) 2, (ii) 3, (iii) 4, (iv) 3, (v) 4, (vi) 5

Question 20: Round up the following upto three significant figures:

- 34.216
- 10.4107
- 0.04597
- 2808

**Answer:** (i) 34.2, (ii) 10.4, (iii) 0.0460 (iv) 2810

Question 21: The following data are obtained when dinitrogen and dioxygen react together to form different compounds:

Mass of dinitrogen | Mass of dioxygen |
---|---|

14 g | 16 g |

14 g | 32 g |

28 g | 32 g |

28 g | 80 g |

(a) Which law of chemical combination is obeyed by the above experimental data? Give its statement.

**Answer:** Law of multiple proportions

(b) Fill in the blanks in the following conversions:

(i) 1 km = ...................... mm = ...................... pm

**Answer:** 1 km = 10^{6} mm = 10^{15} pm

(ii) 1 mg = ...................... kg = ...................... ng

**Answer:** 1 mg = 10^{-6} kg = 10^{6} ng

(iii) 1 mL = ...................... L = ...................... dm^{3}

**Answer:** 1 mL = 10^{-3} L = 10^{-3} dm^{3}

Question 22: If the speed of light is 3.0 × 10^{8} m s^{–1}, calculate the distance covered by light in 2.00 ns.

**Answer:** 1 ns = 10^{-9} s

So, distance in 1 ns

= 3.0 × 10^{8} m × 1^{-1} × 2 × 10^{-9}

= 6.0 × 10^{-1} m = 0.6 m

Question 23: In a reaction

A + B_{2} → AB_{2}

Identify the limiting reagent, if any, in the following reaction mixtures.

- 300 atoms of A + 200 molecules of B
- 2 mol A + 3 mol B
- 100 atoms of A + 100 molecules of B
- 5 mol A + 2.5 mol B
- 2.5 mol A + 5 mol B

**Answer:** As per equation, 1 atom of A reacts with 1 molecule of B

So, in case (i) 200 molecules of B will react with 200 atoms of A. This will leave 100 atoms of A. This means, A is the limiting reagent in this situation. Similarly, limiting reagents in other options are as follows:

(ii) B, (iii) none (iv) A, (v) B

Question 24: Dinitrogen and dihydrogen react with each other to produce ammonia according to the following chemical equation:

N_{2} (g) + 3H_{2} (g) → 2NH_{3} (g)

- Calculate the mass of ammonia produced if 2.00 × 10
^{3}g dinitrogen reacts with 1.00 × 10^{3}g of dihydrogen. - Will any of the two reactants remain unreacted?
- If yes, which one and what would be its mass?

**Answer:** 1 mole of nitrogen (28 g) reacts with 3 mole of hydrogen (6 g)

So, 2000 g nitrogen will react with `6/(28)×2000=428.6` g hydrogen

It will produce 2428.6 g ammonia

This will leave 1000 – 428.6 = 571.4 g hydrogen unreacted

Question 25: How are 0.50 mol Na_{2}CO_{3} and 0.50 M Na_{2}CO_{3} different?

**Answer:** Molar mass of sodium carbonate

= 2 × 23 + 12 + 3 × 16 = 46 + 12 + 48 = 106 g

0.5 mol of sodium carbonate = 106 ÷ 2 = 53 g

0.5 M means 1 liter solution contains 0.53 g of sodium carbonate.

Question 26: If 10 volumes of dihydrogen gas react with five volumes of dioxygen gas, how many volumes of water vapour would be produced?

**Answer:** Following equation shows reaction between hydrogen and oxygen to produce water

2H_{2} + O_{2} → 2H_{2}

It means that 2 volume of hydrogen reacts with 1 volume of oxygen to produce 2 volume of water

So, 10 volume of water vapour would be produced

Question 27: Convert the following into basic units:

(i) 28.7 pm

**Answer:** 28.7 pm = 28.7 × 10^{-12} m = 2.87 × 10^{-11} m

(ii) 15.15 μs

**Answer:** 15.15 μs = 15.15 × 10^{-6} s = 1.515 × 10^{-5} s

(iii) 25365 mg

**Answer:** 25365 mg = 25365 × 10^{-6} kg = 2.5365 × 10^{-2} kg

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