Question

what is the correct solution setup for determining the percent by mass concentration of a 775 mg nabr dissolved in 1015 mg of water?
○ % mass = \\(\frac{775\text{mg nabr}}{1790\text{mg solution}}\\) × 100

○ % mass = \\(\frac{775\text{mg nabr}}{1.790\text{g solution}}\\) × 100

○ % mass = \\(\frac{775\text{mg nabr}}{1015\text{mg water}}\\) × 100

○ % mass = \\(\frac{1015\text{mg water}}{1790\text{mg solution}}\\) × 100

Explanation:

Step1: Recall percent by mass formula

The formula for percent by mass is $\% \text{mass} = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100$. The solute is NaBr (775 mg), and the solution mass is solute + solvent (water). So solution mass = 775 mg + 1015 mg = 1790 mg.

Step2: Analyze each option

• Option 1: Uses $\frac{775\text{mg NaBr}}{1790\text{mg solution}} \times 100$, which matches the formula (solute mass over solution mass, times 100).
• Option 2: Solution mass is in grams (1.790 g) but solute is in mg, units inconsistent.
• Option 3: Divides by water (solvent) mass, not solution mass, incorrect.
• Option 4: Divides water (solvent) mass by solution mass, calculates solvent percent, not solute.

Answer:

A. $\% \text{mass} = \frac{775\text{mg NaBr}}{1790\text{mg solution}} \times 100$ (the first option with 1790mg solution in the denominator)