Question

Calculate the pH of the resulting solution if 20.0 mL of 0.200 M HCl(aq) is added to 30.0 mL of 0.200 M NaOH(aq).

Calculate the pH of the resulting solution if 20.0 mL of 0.200 M HCl(aq) is added to 10.0 mL of 0.300 M NaOH(aq).

Answer 1

(I) effective molarity is given by -

$MV=M_1V_1-M_2V_2$

Where

$$M_1=0.2,M_2=0.2,V_1=30,V_2=20\ ml$$

Now

$$M(20+30)= 2\\M=0.04$$

Molarity = $OH^-$ ion concentration = 0.04

$$pOH=log(\frac{1}{OH^-})=log(25)= 1.398$$

pH= 14-pOH= 14-1.398=12.602

(II)

Similarly

$MV=M_1V_1-M_2V_2$

$$M(20+10)=20\times0.2-10\times0.3\\M=0.033$$

Here molarity = $H^+$ concentration = 0.033

$$pH= log(\frac{1}{H^+})= log(30)=1.477$$