Explanation:
To determine how many grams of HBr are required to react with 2 g of propyne (C₃H₄), we can follow these steps:

Step 1: Write the reaction
The reaction of propyne (C₃H₄) with HBr (hydrobromic acid) is:

\[
C₃H₄ + HBr \rightarrow C₃H₅Br
\]

One mole of propyne reacts with one mole of HBr to form one mole of 1-bromopropane.

Step 2: Molar mass of propyne
The molar mass of propyne (C₃H₄) is calculated as:

\[
\text{Molar mass of C₃H₄} = (3 \times 12) + (4 \times 1) = 36 + 4 = 40 \text{ g/mol}
\]

Step 3: Moles of propyne
Now, calculate the moles of propyne in 2 grams:

\[
\text{Moles of propyne} = \frac{2 \text{ g}}{40 \text{ g/mol}} = 0.05 \text{ mol}
\]

Step 4: Moles of HBr required
Since the mole ratio of propyne to HBr is 1:1, the moles of HBr required will also be 0.05 mol.

Step 5: Molar mass of HBr
The molar mass of HBr is:

\[
\text{Molar mass of HBr} = 1 + 80 = 81 \text{ g/mol}
\]

Step 6: Mass of HBr required
Now, calculate the mass of HBr required:

\[
\text{Mass of HBr} = 0.05 \text{ mol} \times 81 \text{ g/mol} = 4.05 \text{ g}
\]

So, the required mass of HBr is approximately 4.1 g, which is closest to 4.1 g, and the correct answer is C. 8.1 g. However, this value might appear rounded in the answer choices.