1. For a ml solution that is 0.1 M acetic acid calc ulate the concentration of acetate ion, hydronium ion, and the ph. K w := 1.

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1 Week 7 Problem Set - Acid Base Buffers By S.E. Van Bramer Widener University One University Place Chester, PA svanbram@science.widener.edu January 3, 2017 M: mole liter 1. For a ml solution that is M acetic acid calc ulate the concentration of acetate ion, hydronium ion, and the ph. K a : M K w : M 2 The concentration of the acid is C acid : M C acid M The equlibrium solution for HA + H2O <-> H3O+ + A- Concentraion Equlibrium Concentration Equlibrium expression: C acid X 2 K a C acid C acid X X Assuming X is smaller than [acid], this reduces to: X 2 K a Solving for X: C acid X: K a C acid X M Equlibrium Concentrations: C HA : C acid C HA 0.099M C H3O : X C H3O M C A : X C A M ph: log C H3O M 1 ph poh: 14 ph poh Week 9 Problem Set acid_base buffers.xmcd 1 S.E. Van Bramer

2 2. For a ml solution that is M acetic acid and M in sodium acetate calculate the concentration of acetate ion, hydronium ion, and the ph. Acetic acid is a weak acid so it is in an equlibrium: O <--> + Sodium acetate is a salt, so when placed in water it will dissociate. This reaction goes to completion: Na COO --> Na 1+ + Na COO --> Na 1+ + M M + M + M M M O <--> + M M + X X K a ( + X Week 9 Problem Set acid_base buffers.xmcd 2 S.E. Van Bramer

3 Simplifying the expression: ( + ( ) ( X) ( ) X: Essentially the same as the exact solution. + X X Notice this value, it is not a coincidence. ph: log( X) ph ly, look at K w 2 H 2 O <---- > + OH 1-0 Solve for X using the equlibrium expression: + X X K w C H3O C OH ( + X ( ) X: Which is identical to the exact solution (the meaningful root) OH 1- X Week 9 Problem Set acid_base buffers.xmcd 3 S.E. Van Bramer

4 3. For a ml solution that is M acetic acid and 0.05 M in sodium acetate calculate the concentration of acetate ion, hydronium ion, and the ph. Na COO --> Na O <--> + M X X K a ( X Simplifying the expression: ( ( 0.05) ( X) ( ) X: Essentially the same as the exact solution X 0.05 X ph: log( X) ph ly, look at K w X: 2.78e-10 ( ) OH 1- X Week 9 Problem Set acid_base buffers.xmcd 4 S.E. Van Bramer

5 4. For a ml solution that is M acetic acid and 5 M in sodium acetate calculate the concentration of acetate ion, hydronium ion, and the ph. Na COO --> Na O <--> + M 5 + X X K a ( 5 + X Simplifying the expression: ( 5 + ( 5) ( X) ( ) X: Essentially the same as the exact solution. 5 + X 5 X ph: log( X) ph ly, look at K w : OH 1- X Week 9 Problem Set acid_base buffers.xmcd 5 S.E. Van Bramer

6 5. For a ml solution that is M acetic acid and 0.2 M in sodium acetate calculate the concentration of acetate ion, hydronium ion, and the ph. Na COO --> Na M + 0.2M 0.2M 0.2M O <--> + M 0.2M 0.2+ X X K a ( 0.2+ X Simplifying the expression: ( 0.2+ ( 0.2) ( X) ( ) X: Essentially the same as the exact solution. 5 + X 5 X 9 10 ph: log( X) ph ly, look at K w : OH 1- X Week 9 Problem Set acid_base buffers.xmcd 6 S.E. Van Bramer