I have not named any solution "A" as sodium thiosulfate pentahydrate. I only mentioned sodium thiosulfate pentahydrate as a reactant in the context of your experiment. Let's calculate the molar mass of sodium thiosulfate pentahydrate, which has the chemical formula $\text{Na}_2\text{S}_2\text{O}_3 \cdot 5\text{H}_2\text{O}$. We will use the following approximate atomic masses: Sodium (Na): $22.99 \text{ g/mol}$ Sulfur (S): $32.07 \text{ g/mol}$ Oxygen (O): $16.00 \text{ g/mol}$ Hydrogen (H): $1.008 \text{ g/mol}$ Step 1: Calculate the molar mass of $\text{Na}_2\text{S}_2\text{O}_3$. $$ \text{Molar mass of } \text{Na}_2\text{S}_2\text{O}_3 = (2 \times 22.99) + (2 \times 32.07) + (3 \times 16.00) $$ $$ = 45.98 + 64.14 + 48.00 $$ $$ = 158.12 \text{ g/mol} $$ Step 2: Calculate the molar mass of one water molecule ($\text{H}_2\text{O}$). $$ \text{Molar mass of } \text{H}_2\text{O} = (2 \times 1.008) + (1 \times 16.00) $$ $$ = 2.016 + 16.00 $$ $$ = 18.016 \text{ g/mol} $$ Step 3: Calculate the molar mass of five water molecules ($5\text{H}_2\text{O}$). $$ \text{Molar mass of } 5\text{H}_2\text{O} = 5 \times 18.016 \text{ g/mol} $$ $$ = 90.08 \text{ g/mol} $$ Step 4: Add the molar mass of $\text{Na}_2\text{S}_2\text{O}_3$ and $5\text{H}_2\text{O}$ to get the total molar mass of sodium thiosulfate pentahydrate. $$ \text{Total molar mass} = 158.12 \text{ g/mol} + 90.08 \text{ g/mol} $$ $$ = 248.20 \text{ g/mol} $$ Therefore, the molar mass of sodium thiosulfate pentahydrate is approximately $248.20 \text{ g/mol}$. Your calculation of $248 \text{ g/mol}$ is correct. The value of $82 \text{ g/mol}$ is incorrect for sodium thiosulfate pentahydrate. The final answer is $\boxed{\text{}248.20 \text{ g/mol}\text{}}$