Question

A chemist needs to determine the concentration of a solution of nitric acid, HNO3. She puts 875 mL of the acid in a flask along with a few drops of indicator. She then slowly adds 0.800 M Ba(OH)2 to the flask until the solution turns pink, indicating the equivalence point of the titration. She notes that 285 mL of Ba(OH)2 was needed to reach the equivalence point. Solution map In this titration, the concentration of base is known and can be used to calculate the unknown acid concentration: concentration of base ⟶ moles of base ⟶ moles of acid ⟶ concentration of acid Part A How many moles of Ba(OH)2 are present in 285 mL of 0.800 M Ba(OH)2?

Answer 1

Answer:

• There are 0.288 moles of Ba(OH)₂ in 285 mL of 0.800 M Ba(OH)₂

Explanation:

The specific question is how many moles of Ba(OH)₂ are present in 285 mL of 0.800 M Ba(OH)₂.

In a titration, the determination of the number of moles of the substance whose molarity is known, is the first step of the calculations.

Since you know the molar concentration, and the volume of the substance, in this case the base Ba(OH)₂, you can use the molarity formula to solve for the number of moles:

The name of the variables are:

• M = molarity

• n = number of moles of solute (Ba(OH)₂ in this case)

• V = volume in liters of the solution

Formula:

• M = n / V (in liters) .......... [this is the definition of molarity]

Solve for n:

• n = M × V

Substitute the known values>

• n = 0.285 liter × 0.800 M = 0.228 mole ...... keep the 3 significant figures

Answer: 0.288 moles of Ba(OH)₂.

Note that from the number of moles of one component (the base, in this case), you can calculate the molar concentration of the other component (the acid in this case),  because, at the equivalence point, both acid and base reactants are at the theoretical mole ratio, i.e. both are consumed.