Calculate the pH of a solution formed by mixing 250.0 mL of 0.15 M NH_4 CI with

Question

Calculate the pH of a solution formed by mixing 250.0 mL of 0.15 M NH_4 CI with 100.0 mL of 0.20 M NH_3. The K_b for NH_3 is 1.8 times 10^-5. A 1.00 L buffer solution is 0.150 M in HC_7 H_5 O_2 and 0.250 M in LiC_7 H_5 O_2. Calculate the pH of the solution after the addition of 100.0 mL of 1.00 M HCl. The K_a for HC_7 H_5 O_2 is 6.5 times 10^-5. Determine the mass of sodium formate that must be dissolved in 250 mL of a 1.4 M formic acid solution to prepare a buffer with pH 3.36. K_a HCOOH = 1.8 times 10^-4.

Explanation / Answer

Calculate the pH of a solution formed by mixing 250.0 mL of 0.15 M NH4Cl with 100.0 mL of 0.20 M NH3. The Kb for NH3 is 1.8 x 10^-5

pKa +pKb=14

pKa= 14-pKb=14- [-log(Kb)]=14-4.75=9.25

pH= pKa +log[base]/ [salt]=pKa+ log[NH3]/[NH4Cl]

[NH3]=0.250 L x 0.15 mol/L =0.0375 M

[NH4Cl] =0.1Lx0.2 mol/L= 0.02 M

pH= 9.25+ log (0.02/0.0375)=8.98-------------------ANSWER

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