Question

In a sample of 35.0 g hydrogen sulphide, how many moles are contained?
A) 1.03 mol
B) 2.06 mol
C) 6.18 mol
D) 9.45 mol
E) 11.3 mol

Answer 1

Hint: Chemical formula of hydrogen sulphide is ${H_2}S$. To calculate number of moles in given 35.0 g of hydrogen sulphide, use to the formula: number of moles = $\dfrac{{{\text{Given mass}}}}{{{\text{Molecular Mass}}}}$. For doing the calculation of moles, you also need to calculate the molecular mass of hydrogen sulphide.

Complete step by step solution:
As you know, the chemical formula of hydrogen sulphide is ${H_2}S$.
Thus, molecular mass of ${H_2}S$= ${\text{2 \times Atomic weight of hydrogen + Atomic weight of sulphur}}$
Atomic weight of Hydrogen, $H = 1 g/mol$
Atomic weight of Sulphur, $S = 32 g/mol$
Therefore, molecular mass of ${H_2}S$ = $2 \times 1 + 32 = 34{\text{ }}g/mol$
We are asked to calculate the number of moles contained in a sample of 35.0 g of hydrogen sulphide.
Thus, given mass of ${H_2}S$ $= 35.0 g$
Now, formula to calculate number of moles = $\dfrac{{{\text{Given mass}}}}{{{\text{Molecular Mass}}}}$
Therefore, number of moles of ${H_2}S$ = $\dfrac{{35.0}}{{34}} = 1.03{\text{ }}mol$
Hence, option A is the correct answer.
Additional information: Mole, having symbol mol, is the SI unit to measure the amount of substance. One mole of any substance contains Avogadro's number (${N_A}$) of particles i.e., $6.022 \times {10^{23}}particles$ and mass of one mole of a substance in grams is equivalent to its molar mass. Thus, 34 g of hydrogen sulphide constitutes 1 mole.

So the answer to this question is option A.

Note: One should also have the information regarding the percentage of a particular element in a compound. For this, mass percent of the elements present in a compound is usually calculated.
Mass percent of the elements can be calculated as:
Mass % of an element = $\dfrac{{{\text{mass of that element in the compound}}}}{{{\text{molar mass of the compound}}}} \times 100$
Let us calculate mass percent of the elements present in hydrogen sulphide (${H_2}S$):
Mass % of hydrogen, H in ${H_2}S$ = $\dfrac{{2 \times 1}}{{34}} \times 100 = 5.88{\text{ }}\% $
Mass % of sulphur, S in ${H_2}S$ = $\dfrac{{32}}{{34}} \times 100 = 94.11{\text{ }}\% $