21.  What is the correct scientific notation for 0.00016 ?

A. $$1.6\times 10^{-4}$$

B. $$16\times 10^{-5}$$

C. $$0.16\times 10^{-3}$$

D. cannot be determined.

22.  How many significant digits are there in 0.25 ?

A. 1

B. 2

C. 3

D. cannot be determined.

23.  Which of the following number contains three significant digits ?

A. 0.200

B. 0.030

C. 0.0052

D. 0.002

24.  What is the number of neutrons in $$Zn^{2+}$$ ion (Atomic mass namber = 70) (IITJEE 1979)

A. 34

B. 36

C. 38

D. 40

25.  The same amount of Zinc is treated separately with excess of sulphuric acid and excess of sodium hydroxide. What will be the ratio of volumes of hydrogen evolved ? (IIT JEE 1979)

A. 1 : 1

B. 1 : 2

C. 2 : 1

D. 9 : 4

Explanation :

(i) $$Zn+H_{2}SO_{4}\rightarrow ZnSO_{4}+H_{2}$$

(ii) $$Zn+2NaOH\rightarrow Na_{2}ZnO_{2}+H_{2}$$

26.  2.76 g of silver carbonate on being strongly heated yields a residue weighing (IIT JEE 1979)

A. 2.16 g

B. 2.48 g

C. 2.32 g

D. 2.64 g

Explanation :

$$Ag_{2}CO_{3(s)}\overset{\Delta }{\rightarrow} Ag_{2}O_{(s)}+CO_{2(g)}$$

1 mol         1mol

$$\therefore$$ 0.01 mol        0.01 mol

There fore mass of residue $$(Ag_{2}O)$$ = 0.01 molar mass of $$Ag_{2}O$$

= 0.01 x 232

= 2.32 kg

27.  Find the total number of electrons in one molecule of carbon dioxide.

A. 22

B. 44

C. 66

D. 88

28.  A gaseous mixture contains oxygen and nitrogen in the ratio of 1 : 4 by weight. Therefore, the ratio of their number of molecules is

A. 1 : 4

B. 1 : 8

C. 7 : 32

D. 3 : 16

Explanation :

The ratio by weight $$=\frac{1}{4}$$

$$\therefore$$ Ratio of moles $$=\frac{\frac{1}{32}}{\frac{4}{28}}=\frac{28}{4\times 32}=\frac{7}{32}$$

29.  Identify the incorrect unit conversion factor.

A. $$\frac{1\ cm^{3}}{1\ mL}$$

B. $$\frac{1\ cm}{10\ mm}$$

C. $$\frac{60\ s}{1\ min}$$

D. None of these

30.  $$90g\ KClO_{3}$$ on heating gives $$2.96g\ KCl$$ and 1.92g oxygen. Which of the following laws is illustrated by this statement ?