When excess hydrogen sulfide gas (H_2S) reacts with sulfur(IV) oxide (SO_2) according to t… — JAMB UTME

When excess hydrogen sulfide gas (

H_2S

) reacts with sulfur(IV) oxide (

SO_2

) according to the balanced chemical equation:

2H_2S(g) + SO_2(g) \rightarrow 3S(s) + 2H_2O(l)

What mass of elemental sulfur is precipitated when

5.6\text{ dm}^3

SO_2

gas at STP reacts completely?
(Molar volume of gas at STP =

22.4\text{ dm}^3\text{ mol}^{-1}

; Relative atomic mass:

S = 32

)

The mass of elemental sulfur precipitated is

24.0\text{ g}

First, find the moles of

SO_2

at STP:

\frac{5.6\text{ dm}^3}{22.4\text{ dm}^3\text{ mol}^{-1}} = 0.25\text{ mol}

. According to the balanced equation,

1\text{ mole of } SO_2

produces

3\text{ moles of } S

. Thus,

0.25\text{ mol of } SO_2

produces

0.75\text{ mol of } S

. Multiplying by the relative atomic mass of sulfur (

32\text{ g mol}^{-1}

) gives

0.75 \times 32 = 24.0\text{ g}

Calculate the number of moles of

SO_2

gas reacting at STP.

\text{Moles of } SO_2 = \frac{\text{Volume at STP}}{\text{Molar volume at STP}} = \frac{5.6\text{ dm}^3}{22.4\text{ dm}^3\text{ mol}^{-1}} = 0.25\text{ mol}

Gas volume at STP is converted to moles using the standard molar volume of

22.4\text{ dm}^3\text{ mol}^{-1}

Determine the mole ratio between

SO_2

and

S

from the balanced reaction equation.

From

2H_2S(g) + SO_2(g) \rightarrow 3S(s) + 2H_2O(l)

1\text{ mol of } SO_2

yields

3\text{ mol of } S

Stoichiometric coefficients give the exact molar equivalence between reactants and products.

Calculate the moles of

S

formed.

\text{Moles of } S = 0.25\text{ mol } SO_2 \times 3 = 0.75\text{ mol } S.

Multiplying moles of

SO_2

by the stoichiometric factor of 3 gives the moles of produced sulfur.

Convert the moles of sulfur to mass in grams.

\text{Mass of } S = 0.75\text{ mol} \times 32\text{ g mol}^{-1} = 24.0\text{ g}.

Mass is obtained by multiplying number of moles by relative atomic mass.

Gas stoichiometry at STP and redox reaction between hydrogen sulfide and sulfur(IV) oxide

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