Sodium hypochlorite, NaClO, is added as a disinfectant to the water supply. You
are analyzing a sample from the supplier to verify its purity and you prepare to titrate 250.0 mL of a solution containing 1.86 g of NaClO with a 1.00 M HCl solution.
What is the pH of the solution at the equivalence point?
I must admit this is a new one on me; however, I assume we have HClO at the equivalence point.
moles NaClO = 1.86/about 74 = about 0.025
moles HClO = 0.025 at equivalence point.
How much 1M HCl is required to titrate the sample? That will be M = mmols/mL or
mL = mmols/M = 25 mmoles/1M = 25 mL. The initial volume was 250 mL so the final volume is 250+ 25 = 275 mL.
(HClO) = mmoles/total volume = 25/275 = 0.0909M
..........HClO ==> H^+ + ClO^-
initial...0.0909....0......0
change......-x.......x......x
equil.....0.0909-x...x.......x
Ka = (H^+)(ClO^-)/(HCl0)
Substitute from the ICE chart above into the Ka expression and solve for (H^+), then convert to pH by pH = -log(H^+).
Post your work if you need additional assistance.
To determine the pH of the solution at the equivalence point, we need to understand the reaction that takes place between sodium hypochlorite (NaClO) and hydrochloric acid (HCl). The balanced chemical equation for this reaction is:
NaClO + HCl → NaCl + H2O + Cl2
In this reaction, sodium hypochlorite reacts with hydrochloric acid to produce sodium chloride, water, and chlorine gas.
At the equivalence point, all of the sodium hypochlorite in the solution would have reacted completely with the hydrochloric acid, resulting in equal amounts of moles of NaClO and HCl in the solution. Since the reaction between NaClO and HCl is a 1:1 ratio, the moles of HCl would be equal to the moles of NaClO.
Now let's calculate the number of moles of NaClO in the given 1.86 g:
Molar mass of NaClO = 22.99 g/mol (sodium) + 35.45 g/mol (chlorine) + 16.00 g/mol (oxygen) = 74.44 g/mol
Number of moles of NaClO = mass of NaClO / molar mass of NaClO
= 1.86 g / 74.44 g/mol
≈ 0.0250 mol NaClO
Since the ratio between NaClO and HCl is 1:1, we know that the number of moles of HCl is also 0.0250 mol.
Now, let's determine the volume of the HCl solution required to reach the equivalence point. Since the Molarity of HCl is given as 1.00 M, we can use the formula:
Molarity (M) = moles / volume (L)
Rearranging the equation to solve for volume:
Volume (L) = moles / Molarity
= 0.0250 mol / 1.00 mol/L
= 0.0250 L
= 25.0 mL
Therefore, it would take 25.0 mL of 1.00 M HCl solution to reach the equivalence point.
At the equivalence point, the moles of NaClO would have reacted completely with the moles of HCl, resulting in the formation of NaCl and H2O. Since NaCl and H2O are both neutral compounds, the pH at the equivalence point would be around 7 (neutral).