Answer to Question #114598 in General Chemistry for Nicolas

Question

7. A 24.6 ml solution of unknown acid contains 0.0034 mols of acid (5.03 g/L) in solution.
Calculate the molar mass of the solution, and use that to calculate the concentration and
pH of the solution.
8.) Which of the 4 acids was the acid from problem 7?
a) H2SO4
b) HCl
c) HNO3
d) HI

Answer 1

Solution:

The molar mass of unknown acid = (Density of solution × Volume of solution) / (Moles of acid).

Units: [g/mol] = (g/L × L) / (mol) = [g/mol] (Correct).

Then,

The molar mass of unknown acid = (5.03 g/L × 0.0246 L) / (0.0034 mol) = 36.39 g/mol

The molar mass of H₂SO₄ is 98.079 g/mol.

The molar mass of HCl is 36.46 g/mol.

The molar mass of HNO₃ is 63.01 g/mol.

The molar mass of HI is 127.911 g/mol.

The molar mass of unknown acid = 36.39 g/mol.

Therefore, the unknown acid is HCl.

The mass of solution = Density of solution × Volume of solution

The mass of solution = (5.03 g/L) × (0.0246 L) = 0.1237 g

Molarity of HCl = Moles of HCl / Volume of solution

Molarity of HCl = (0.0034 mol) / (0.0246 L) = 0.1382 mol/L

Molarity of HCl = 0.1382 mol/L

Hydrochloric acid, HCl, is a strong acid that dissociates completely in water to form H₃O⁺ and Cl^–.

HCl(aq) + H₂O(l) ⇄ H₃O⁺(aq) + Cl^–(aq).

Molarity of HCl = C_M(HCl) = [H₃O⁺] = 0.1382 mol/L.

[H₃O⁺] = 0.1382 M

The pH is calculated using the expression:

pH = - log[H₃O⁺]

pH = - log(0.1382) = 0.86

pH = 0.86

Answer (7):

The molar mass of unknown acid = 36.39 g/mol

Molarity of HCl = 0.1382 M

pH = 0.86

Answer (8):

The unknown acid is HCl.