Determine the mass percent (m/m) of each: a) 25 g of KCl and 125 g of H2O b) 75 g NaOH in a 325 g NaOH solution
The Correct Answer and Explanation is:
To determine the mass percent (mass/mass percent, or % m/m), use the following formula:Mass percent=(mass of solutemass of solution)×100\text{Mass percent} = \left( \frac{\text{mass of solute}}{\text{mass of solution}} \right) \times 100Mass percent=(mass of solutionmass of solute)×100
a) 25 g of KCl and 125 g of H₂O
Step 1: Determine the total mass of the solution.Mass of solution=mass of solute (KCl)+mass of solvent (H₂O)=25 g+125 g=150 g\text{Mass of solution} = \text{mass of solute (KCl)} + \text{mass of solvent (H₂O)} = 25\ \text{g} + 125\ \text{g} = 150\ \text{g}Mass of solution=mass of solute (KCl)+mass of solvent (H₂O)=25 g+125 g=150 g
Step 2: Use the formula to calculate mass percent.Mass percent of KCl=(25150)×100=16.67%\text{Mass percent of KCl} = \left( \frac{25}{150} \right) \times 100 = 16.67\%Mass percent of KCl=(15025)×100=16.67%
b) 75 g NaOH in a 325 g NaOH solution
In this case, the mass of the solution is already given as 325 g, and the solute (NaOH) is 75 g.
Step 1: Apply the formula.Mass percent of NaOH=(75325)×100=23.08%\text{Mass percent of NaOH} = \left( \frac{75}{325} \right) \times 100 = 23.08\%Mass percent of NaOH=(32575)×100=23.08%
Explanation
Mass percent (also called mass/mass percent or % m/m) is a way to express the concentration of a solution. It shows the ratio of the mass of the solute to the total mass of the solution, multiplied by 100 to convert it into a percentage. This form of concentration is especially useful when dealing with solid-liquid or liquid-liquid mixtures in a laboratory or industrial context.
To calculate mass percent, it is essential first to understand the two components of a solution: the solute and the solvent. The solute is the substance being dissolved, while the solvent is the medium in which the solute dissolves. The total mass of the solution is the sum of the solute and solvent masses.
In part (a), potassium chloride (KCl) is dissolved in water (H₂O). Adding their masses gives a total of 150 grams for the solution. The 25 grams of KCl represent the solute. Using the formula, the concentration of KCl in the solution is 16.67%.
In part (b), the total mass of the solution is given as 325 grams, which already includes the 75 grams of sodium hydroxide (NaOH) as the solute. Therefore, the percentage concentration is found directly using the formula. The result shows that NaOH makes up 23.08% of the solution by mass.
Mass percent is particularly valuable in preparing solutions where precise concentrations are necessary, such as in chemistry labs, pharmaceuticals, and industrial formulations. It allows for easy communication of how much solute is present in a given quantity of solution.
