Calculate the number of moles of solute present in each of the following solutions: (a) 245 mL of 1.50 M HNO3; (b) 50.0 mg of an aqueous solution that is 1.25 m NaCl.

Steps to Solve

1. Calculate Moles of HNO₃ To find the number of moles of HNO₃, we use the formula: $$ \text{moles} = \text{molarity} \times \text{volume (L)} $$ Convert 245 mL to liters: $$ 245 , \text{mL} = 0.245 , \text{L} $$ Now substitute the values into the formula: $$ \text{moles of HNO}_3 = 1.50 , \text{M} \times 0.245 , \text{L} $$

2. Calculate Moles of NaCl For NaCl, we start by converting the mass from milligrams to grams: $$ 50.0 , \text{mg} = 0.0500 , \text{g} $$ Next, we convert grams to moles using the formula: $$ \text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}} $$ The molar mass of NaCl is approximately 58.44 g/mol. Therefore: $$ \text{moles of NaCl} = \frac{0.0500 , \text{g}}{58.44 , \text{g/mol}} $$ Finally, since the concentration is molality (m), we must consider the density of the solution if needed; however, in this case, we calculate the moles directly from the mass.

3. Finalizing the Calculations Calculating both moles:

• For HNO₃: $$ \text{moles of HNO}_3 = 1.50 \times 0.245 = 0.3675 , \text{moles} $$

• For NaCl: $$ \text{moles of NaCl} = \frac{0.0500}{58.44} \approx 0.000856 , \text{moles} $$