1

### NEET 2017

Which of the following is dependent on temperature?
A
Molarity
B
Mole fraction
C
Weight percentage
D
Molality

## Explanation

Molarity =
Number of moles of solute
Volume of solution (in L)

Molality =
Number of moles of solute
Mass of solvent (in kg)

Mole fraction =
Number of moles of component
Total number of moles of all components

Weight percentage =
Weight of a component
Total weight of solution
$\times$ 100

KEY Concept : Those units which are volume related will be affected by the change in temperature.

The definition of molarity is - Number of moles of solute present in one litre of solution. From definition you can see it is depends on volume which increases with increasing temperature and decreases with decreasing temperature.

All the other three options (Molality, Mole fraction, Weight fraction of solute) are mass related units and temperature has no effect on mass.
2

### NEET 2016 Phase 2

Suppose the elements X and Y combine to form two compounds XY2 and X3Y2. When 0.1 mole of XY2 weights 10 g and 0.05 mole of X3Y2 weighs 9 g, the atomic weights of X and Y are
A
40, 30
B
60, 40
C
20, 30
D
30, 20

## Explanation

Assume the atomic weight of element X is x and element Y is y.

For XY2, $n = {w \over {Mol.\,\,wt.}}$

$0.1 = {{10} \over {x + 2y}}$

$\Rightarrow x + 2y = {{10} \over {0.1}} = 100$    ...(i)

For X3Y2, $n = {w \over {Mol.\,\,wt.}}$

$0.05 = {9 \over {3x + 2y}}$

$\Rightarrow 3x + 2y = {9 \over {0.05}} = 180$    ...(ii)

On solving equations (i) and (ii),

we get y = 30 g mol-1

Put the value of y in equation. (i)

x + 2(30) = 100

$\Rightarrow$ x = 100 - 60 = 40 g mol-1
3

### AIPMT 2015 Cancelled Paper

A mixture of gases contains H2 and O2 gases in the ratio of 1 : 4 (w/w). What is the molar ratio of the two gases in the mixture ?
A
16 : 1
B
2 : 1
C
1 : 4
D
4 : 1

## Explanation

Ratio of weight of gases = wH2 : wO2 = 1 : 4

$\therefore$ Number of moles of H2 = $1 \over 2$

$\therefore$ Number of moles of O2 = $4 \over 32$

Ratio of moles of gases

= nH2 : nO2

= $1 \over 2$ : $4 \over 32$

= $1 \over 2$ $\times$ $32 \over 4$ = 4 : 1
4

### AIPMT 2015

The number of water molecules is maximum in
A
1.8 gram of water
B
18 gram of water
C
18 moles of water
D
18 molecules of water

## Explanation

1.8 g of water = ${{6.023 \times {{10}^{23}}} \over {18}} \times 1.8$

= 6.023 $\times$ 1022 molecules

$\therefore$ 18 g of water = 6.023 $\times$ 1023 molecules = 1 mole of water

$\therefore$ 18 moles of water = 18 $\times$ 6.023 $\times$ 1023 molecules

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