select the correct answer. a laboratory technician needs to prepare 500 milliliters of a 2 m solution of…

select the correct answer. a laboratory technician needs to prepare 500 milliliters of a 2 m solution of sodium hydroxide (naoh) in a flask. the flask has a volume of exactly 500 milliliters. which quantity does the laboratory technician need to measure before making the solution? a. the mass of the solute b. the mass of the solvent c. the mass of the solution d. the mass of the flask

select the correct answer. a laboratory technician needs to prepare 500 milliliters of a 2 m solution of sodium hydroxide (naoh) in a flask. the flask has a volume of exactly 500 milliliters. which quantity does the laboratory technician need to measure before making the solution? a. the mass of the solute b. the mass of the solvent c. the mass of the solution d. the mass of the flask

Answer

Explanation:

Step1: Recall solution - making concept

To prepare a solution of a known molarity (M) and volume, we use the formula $n = M\times V$ (where $n$ is the number of moles, $M$ is molarity, and $V$ is volume in liters) to find the number of moles of the solute. Then, to get the actual amount of solute to measure out, we convert moles to mass using the molar - mass of the solute.

Step2: Analyze the options

  • Option A: The mass of the solute is correct. We need to know the mass of sodium hydroxide (the solute) to prepare the solution. First, we calculate the number of moles of NaOH using $n = M\times V$ ($V = 500\ mL=0.5\ L$, $M = 2\ M$), then convert moles to mass using the molar mass of NaOH ($M_{NaOH}=40\ g/mol$).
  • Option B: The mass of the solvent is not the first - measured quantity. After calculating the mass of the solute, we add the appropriate amount of solvent to make the final volume of the solution.
  • Option C: The mass of the solution is not the quantity we measure first. We prepare the solution by measuring the solute and adding solvent, and the mass of the solution is a result of these operations.
  • Option D: The mass of the flask is not relevant to preparing the solution in terms of the chemical quantities.

Answer:

A. the mass of the solute