ChE Exam

Analytical Chemistry

Question 1 (10 points)

A `0.500-g` sample containing `Na₂CO₃` plus inert matter is analyzed by adding `50.0 mL` of `0.100 M HCl`, a slight excess, boiling to remove `CO₂`, and then back-titrating the excess acid with `0.100 M NaOH`. If `5.6 mL NaOH` is required for the back-titration, what is the percent `Na₂CO₃` in the sample?

Answer: 6.13%

Question 2 (10 points)

A `425.2-mg` sample of a purified monoprotic organic acid is titrated with `0.1027 M NaOH`, requiring `28.78 mL`. What is the formula weight of the acid?

Answer: 84.4%

Question 3 (40 points)

`A` and `B` react as follows: `A + B ⇄ C + D`. The equilibrium constant is `2.0 × 10³`. If `0.30 mol` of `A` and `0.80 mol` of `B` are mixed in `1 L`, what are the concentrations of `A`, `B`, `C`, and `D` after reaction?

Answer: [A] = 9.0 × 10^-5 M, [B] = 0.50 M, [C] = [D] = 0.30 − x ≈ 0.30 M

Question 4 (30 points)

Calculate the activity coefficients of each ion in a solution containing `0.0020 M Na₂SO₄` and `0.0010 M Al₂(SO₄)₃`.

Answer: f_Na⁺ = 0.867, f_SO₄^2- = 0.56, f_Al³⁺ = 0.35

Question 5 (10 points)

`V_a mL` of a strong acid solution of `pH 2.00` is mixed with `V_b mL` of a strong base solution of `pH 11.00`. Express `V_a` in terms of `V_b` if the mixture is neutral. The solution temperature is 24 °C.

Answer: V_a = 0.10V_b

Question 6 (10 points)

A student weighed out `0.6529 g` of anhydrous monohydrogen sodium phosphate `(Na₂HPO₄)` and `0.2477 g` of dihydrogen sodium phosphate `(NaH₂PO₄H₂O)`, then dissolved them into `100 mL` of distilled water. What is the `pH`?

Answer: 7.53