Sodium hypochlorite,NaOCl,is the active ingredient in household bleach.What is the concentration of hypochlorite ion if 20.00 mL of bleach requires 32.00 mL of 0.500 mol L^-1 HCl(aq)to reach the equivalence point? A)0.300 mol L^-1 B)0.312 mol L^-1 C)0.800 mol L^-1 D)1.30 mol L^-1

Question

Quizplus · Accepted Answer

First we need to write out the balanced equation for the reaction:

NaOCl + HCl → NaCl + H2O + Cl2

From this equation, we can see that the hypochlorite ion (OCl-) reacts with the H+ ion from the HCl to produce Cl2 gas. At the equivalence point, all of the H+ ions from the HCl have reacted with OCl- ions, so we know that the moles of H+ ions in the HCl is equal to the moles of OCl- ions in the bleach. We can use this information to calculate the concentration of OCl- in the bleach:

n(H+) = c(H+) × V(H+) = c(OCl-) × V(OCl-)

c(OCl-) = n(H+) / V(OCl-) = c(H+) × V(H+) / V(OCl-)

We know that V(H+) = 32.00 mL and c(H+) = 0.500 mol/L, and we can assume that the volume of bleach (V(OCl-)) is the same as the volume of HCl used, which is 32.00 mL. Plugging in these values gives:

c(OCl-) = 0.500 mol/L × (32.00 mL/1000 mL) / (20.00 mL/1000 mL) = 0.800 mol/L

This is the concentration of OCl-, so to get the concentration of hypochlorite ion, we need to multiply by the ratio of the molar masses:

c(hypochlorite ion) = c(OCl-) × (mass of OCl- / mass of hypochlorite ion)

The molar mass of OCl- is 22.99 g/mol (sodium: 22.99 g/mol; oxygen: 15.99 g/mol; chlorine: 35.45 g/mol) and the molar mass of hypochlorite ion (OCl-) is 52.46 g/mol (sodium: 22.99 g/mol; oxygen: 15.99 g/mol; chlorine: 35.45 g/mol - 1 electron). The ratio of these masses is:

mass of OCl- / mass of hypochlorite ion = 22.99 g/mol / 52.46 g/mol = 0.438

So:

c(hypochlorite ion) = 0.800 mol/L × 0.438 = 0.350 mol/L

Therefore, the concentration of hypochlorite ion in the bleach is 0.350 mol/L, which is closest to answer choice C (0.800 mol/L is the concentration of OCl-, not hypochlorite ion).

Sodium Hypochlorite,NaOCl,is the Active Ingredient in Household Bleach