Lecture 16 - Consider a solution(Solution A of 0.250 M...

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2/12/10 1 Consider a solution (Solution A) of 0.250 M HCOOH (formic acid). If K a = 1.8 ! 10 -4 , what is the percent dissociation? A) 2.64 % B) 0.66 % C) 2.68 % D) 2.18 % E) 25.00% 90 Step 1. List all relevant reactions and K values K a >> K w , i.e., stronger acid that H 2 O HCOOH(aq) + H 2 O(l) ! HCOO - (aq) + H 3 O + (aq) Water autoionization Solving the quadratic Making small K approximation 91 Step 3. Substitute into expression for K a and solve for x. Step 2. Make up I.C.E. table for dominant reaction. I C E 0.250 M 0.0 M 0.0 M +x - x + x 0.250 - x +x + x x = 6.6 ! 10 -3 = [H 3 O + ] K a = 1.8 ! 10 " 4 = HCOO - [ ] H 3 O + [ ] HCOOH [ ] = (x)(x) (0.250 - x) pH= 2.18 HCOOH(aq) + H 2 O(l) ! HCOO - (aq) + H 3 O + (aq) Solve quadratic: Could use small K approximation % dissoc = 6.6 ! 10 -3 0.250 ! 100 = 2.64% Consider a solution (Solution B) of 0.100 M HCOONa (sodium formate). What is the pH of the solution? A) 5.63
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Lecture 16 - Consider a solution(Solution A of 0.250 M...

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