If you have 500.0 mL of a 0.10 M solution of HClO, what mass of NaClO (molar mass = 74.44 g/mol) do you need to add to make a buffer with a pH of 7.5?

• A. 7.44 g
• B. 3.72 g
• C. 1.86 g
• D. 0.93 g

Answer: (B)

Verified step by step guidance

1. First, understand that you are creating a buffer solution using HClO and NaClO. A buffer solution resists changes in pH when small amounts of acid or base are added.
2. Use the Henderson-Hasselbalch equation to find the ratio of the concentrations of the conjugate base (ClO⁻) and the weak acid (HClO). The equation is: ${\mathrm{pH}}_{}={\mathrm{pK}}_{}+log\left(\frac{\mathrm{[ClO⁻]}}{\mathrm{[HClO]}}\right)$
3. Determine the ${\mathrm{pK}}_a$ of HClO. You can find this value in a chemistry reference table. For HClO, ${\mathrm{pK}}_a$ is approximately 7.5.
4. Since the desired pH is 7.5, set up the equation: $7.5=7.5+log\left(\frac{\mathrm{[ClO⁻]}}{\mathrm{[HClO]}}\right)$. This simplifies to $log\left(\frac{\mathrm{[ClO⁻]}}{\mathrm{[HClO]}}\right)=0$, meaning $\mathrm{[ClO⁻]}=\mathrm{[HClO]}$.
5. Calculate the mass of NaClO needed. Since $\mathrm{[ClO⁻]}=\mathrm{[HClO]}$, the concentration of ClO⁻ should be 0.10 M in 500.0 mL. Use the formula: $\mathrm{mass}=\mathrm{molarity}\times \mathrm{volume}\times \mathrm{molar\; mass}$ to find the mass of NaClO needed.