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Standard XII

Chemistry

Concentration Terms

PbNO 3 2 an...

Question

Pb(NO $_{3}$ ) $_{2}$ and Kl reacts in aqueous solution to form an yellow precipitate of PbI $_{2}$ . In one series of experiments, the masses of two reactants varied but the total mass of the two was held constant at $5.0$ g. What maximum mass in grams of PbI $_{2}$ can be produced in the above experiment? (Write your answer to the nearest integer.)

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Solution

$P b (N O_{3})_{2} + 2 K I \to P b I_{2} + 2 K N O_{3}$
The molar masses of $P b (N O_{3})_{2}$ and KI are 331.2 g/mol and 166 g/mol respectively. The mass of $P b I_{2}$ is 461 g/mol.
Let X moles of $P b (N O_{3})_{2}$ react with 2X moles of KI to produce X moles of $P b I_{2}$ .
Total mass of $P b (N O_{3})_{2}$ and KI that has reacted is $331.2 X + 2 \times 166 \times X = 663.2 X$ g. The total mass of two was held constant at 5.0 g

$633.2 X = 5$
$X = 0.007539$ moles.
This is the number of moles of $P b I_{2}$ obtained. The mass of $P b I_{2}$ obtained $= 0.007539 m o l \times 461 g / m o l = 3.4 g$ .
The answer to the nearest integer is 3.

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C a {C l}_{2} (a q) + {N a}_{2} {C O}_{3} (a q) \to C a {C O}_{3} (s) + 2 N a C l (a q)

I

2.50 g

C a {C l}_{2}

is fully dissolved in a beaker of water and

2.50 g

{N a}_{2} {C O}_{3}

is fully dissolved in water in a second beaker. The two solutions are mixed to form a

C a {C O}_{3}

precipitate and aqueous

N a C l . {N a}_{2} {C O}_{3}

will be the limiting reactant in this experiment.

I I

. Sodium carbonate has a molar mass which is less than the molar mass of calcium chloride.

Q. 4.0 g of a mixture of

N a C l

and an unknown metal iodide

M I_{2}

was dissolved in a water to form its aqueous solution. To this aqueous solution, The aqueous solution

A g N O_{3}

was added gradually so that silver halides are precipitated. The precipitates were weighed at regular interval and the given curve for the mass of precipitate versus volume of

A g N O_{3}

added was obtained. With the knowledge of the fact that halides are precipitated successively i.e. when one halide is precipitating , the other halide remains in the solution, answer the following questions:
(Molar mass of

A g = 108, I = 127, N a = 23

).
Points

P

and

Q

are:

P (20, 4.8), Q (37, 7.25)

What is the approximate mass percentage of

M I_{2}

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

N₂₍_g₎ + H₂₍_g₎ → 2NH₃₍_g₎

(i) Calculate the mass of ammonia produced if 2.00 × 10³ g dinitrogen reacts with 1.00 × 10³ g of dihydrogen.

(ii) Will any of the two reactants remain unreacted?

(iii) If yes, which one and what would be its mass?

Q. To determine the

N a C l

content in commercial

N a O H

2 g

of the latter was dissolved in water and an excess amount of an

A g N O_{3}

solution was added to this solution. The precipitate formed was washed and dried. Its mass was

0.287 g

. Find the mass of

N a C l

in the initial sample.

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