Question

- What is a primary standard? You have weighed out precisely 2.471 g of oxalic acid dihydrate, and diluted it to 250,0 mL. What is its molarity? (Hint: Example 12.1) What is a secondary standard? You have diluted 9 mL of roughly 6 M NaOH to 500 mL. What is the approximate molarity of the base? (Pay attention to significant figures) (Hint: Example 12.2) What is the name of the indicator you are to use and what color does it turn at the end point?

Answer 1

(1)A primary standard is reagent which is very pure. This standard is a reference chemical used to measure an unknown concentration of another known chemical.

(2) Mass of oxalic acid dihydrate = 2.471 g

Molar mass of oxalic acid dihydrate = 126.065 g/mol

Number of moles of oxalic acid dihydrate = Mass in gram / Molar mass = 2.471 g / 126.065 g/mol = 0.0196 mol

Volume of solution = 250.0 ml = 250.0 L / 1000 = 0.250 L

Molarity = number of moles of solute / Molar mass

Molarity = 0.0196 mol / 0.250 L = 0.0784 mol/L = 0.0784 M

(3) A secondary standard is a standard that is prepared in the laboratory for a specific analysis. It is generally standardized against a primary standard.

(4) Concentration of NaOH = 6 M = 6 mol/L

Volume of NaOH solution = 9 ml = 9 L / 1000 = 0.009 L

Number of moles of NaOH = 6 mol/L * 0.009 L = 0.054 mol

Final volume of solution = 500 ml = 500 L / 1000 = 0.500 L

Molarity of the base = 0.054 mol / 0.500 L = 0.108 mol/L = 0.108 M