A student adds 25.00mL of distilled water to a 10.00mL sample of HCl of an unknown molarity and titrates this against a 0.085M NaOH solution. Will the addition of the water interfere with the determination of the molarity of the HCl sample? Why or why not?
No, it will not.
The whole concept of a titration is that the indicator will turn when mols base = mols acid. Add water does not change the number of mols acid or base. Remember mols acid or base = M x L = mols.
When I demonstrated a titration to my students I always used water liberally to wash down the sides of the titration flask. And, of course, waited for the obvious remark of "but doesn't that dilute the acid in the flask" to which I always replied, "yes it does but doesn't it dilute the base at the same time?" So the answer is either
1. Yes, it dilutes the acid but it also dilutes the base so the result evens out, or,
2. It doesn't matter since mols acid = mols base is when the indicator changes and adding water doesn't change mols acid or base.
To determine whether the addition of water would interfere with the determination of the molarity of the HCl sample, we need to consider the principles of titration.
In a titration, a known concentration solution (the titrant) is slowly added to react with the analyte until its stoichiometric equivalence point is reached. The equivalence point is the point in the titration where the reactants are present in stoichiometrically equivalent amounts.
The addition of distilled water to the HCl sample results in dilution of the HCl solution. Dilution occurs when additional solvent (water, in this case) is added to decrease the concentration of the solute (HCl).
In this case, the dilution of the HCl sample will not interfere with the determination of its molarity. The reason for this is that the concentration of the solute (HCl) remains unchanged by the addition of water. However, the total volume of the solution (HCl + water) increases.
During the titration, we are determining the equivalence point, which is based on the stoichiometry of the reaction. The molarity of the HCl solution is determined by comparing the volume of the titrant (NaOH) required to reach the equivalence point with the balanced chemical equation of the reaction.
Since the addition of water does not change the concentration of HCl, the stoichiometry of the reaction between HCl and NaOH remains the same. Therefore, the addition of water will not interfere with the determination of the molarity of the HCl sample.
To calculate the molarity of the HCl sample, you need to measure the volume of NaOH required to reach the equivalence point. Using the balanced chemical equation and the known concentration of the NaOH solution (0.085M), you can determine the moles of NaOH used. Then, by applying stoichiometry, you can determine the moles of HCl present in the original 10.00mL sample. Finally, divide the moles of HCl by the initial volume (in liters) of the HCl solution to get the molarity.