A student needed to standardize a solution of NaOH which was approximately 0.125 M. The student carefully prepared the titration setup, but after 25 mL of NaOH was added, what was the pH of solution if initially flask contains 25 mL of 0.250 M HCl solution?

is the equation NaOH + HCl-> NaCl + H2O?

I do not understand "Subtract the obtain the HCl concn." How do I calculate the HCl concn?

To determine the pH of the solution after adding 25 mL of NaOH to 25 mL of 0.250 M HCl, we need to consider the reaction that occurs during the titration. NaOH is a strong base, and HCl is a strong acid. When they react, they form water (H2O) and sodium chloride (NaCl).

The balanced chemical equation for the reaction between NaOH and HCl is:

NaOH + HCl -> H2O + NaCl

From the equation, we can see that for every 1 mole of NaOH, 1 mole of HCl is required for complete reaction. Therefore, the moles of NaOH added can be calculated using its molarity and volume, as follows:

Moles of NaOH = Molarity of NaOH * Volume of NaOH (in L)
= 0.125 M * 0.025 L
= 0.003125 moles

Since the reaction is 1:1, the moles of HCl consumed during the reaction will also be 0.003125 moles.

To find the concentration of HCl after the reaction, subtract the moles of HCl consumed from the initial concentration:

Final concentration of HCl = (Initial concentration of HCl * Initial volume of HCl - moles of HCl consumed) / Final volume of solution

The initial volume of HCl is 25 mL, and the final volume of the solution is the sum of the volumes of NaOH and HCl, which is 50 mL.

Final concentration of HCl = (0.250 M * 0.025 L - 0.003125 moles) / 0.050 L
= 0.24375 M

Now that we have the concentration of HCl in the final solution, we can determine the pH. Since HCl is a strong acid, it dissociates completely in water, and the concentration of H+ ions is equal to the concentration of HCl.

pH = -log[H+]
= -log(0.24375)
≈ 0.612

Therefore, the pH of the solution after adding 25 mL of NaOH to 25 mL of 0.250 M HCl is approximately 0.612.

Write the equation and balance it. Then calculate the number of moles HCl initially, then number of moles NaOH added after 25 mL. Subtract the obtain the HCl concn and pH from there.

You subtract the moles of NaoH from the moles of HCl to determine the excess moles left, which decides the pH.

To find pH,
pH= -log(concent. of H+) or ( concen. of OH-)
If NaOH is more than HCl, you get find pH by subtracting the pOH you get from the above formula from 14.