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Equilibrium constant, Ka

Equilibrium constant, Ka . By Soo Jeon. background. Most weak acids dissociate 5% or less, that is, 95% or more of the acid remains as HA. The smaller the value of Ka, the weaker the acid. Background.

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Equilibrium constant, Ka

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  1. Equilibrium constant, Ka By Soo Jeon

  2. background • Most weak acids dissociate 5% or less, that is, 95% or more of the acid remains as HA. The smaller the value of Ka, the weaker the acid.

  3. Background .. • Monoprotic acids are those acids that are able to donate one proton per molecule during the process of dissociation(sometimes called ionization) as shown below (symbolized by HA): HA(aq)  +  H2O(l)  ->     H3O+(aq)  +  A-(aq)

  4. scenario • A student prepared three solutions by adding 12.00, 18.5, and 35.00ml of 7.5x10-2M NaOH solution to 50.00ml of a 0.100M solution of a weak monoprotic acid, HA. The solutions were labeled X,Y,and Z respectively. Each of the solutions were diluted to a total volume of 100.00ml with distilled water. The pH readings of these solutions, obtained through the use of a calibrated pH meter, were X,Y, Z. We are going to determine Ka for a monoprotic weak acid.

  5. Purpose • This experiment will determine Ka for a monoprotic weak acid using pH value and A-ion concentration.

  6. Materials • 7.5x10-2M NaOH solution • 0.100M solution of a weak monoprotic acid • distilled water • calibrated pH meter • 100 ml of Three beakers

  7. Using the calibrated pH meter, find pH of each solution. X : 6.50 Y : 6.70 Z : 7.10 pH of each solution

  8. Calculations • Convert pH to equivalent H30+ concentration. 1) pH = -log [H+] [H30+] = 10-pH • X : 10-6.50 = 3.16 x 10-7M H30+ • Y : 10-6.70 = 2.00 x 10-7M H30+ • Z : 10-7.10 = 7.94 x 10-8M H30+

  9. 2. Calculate the number of moles of acid added • The moles of acid was constant for each solution. 50.00ml x 1L x 0.100 moles HA 1 1000ml 1 L = 5.00 x 10-3 mol HA

  10. 3. Calculate the number of moles of OH- added • X : 12.00ml x 1L x 7.50 x 10-2 mol OH- 1 1000ml 1 L = 9.00 x 10-4 mol OH- • Y : 18.50ml x 1L x 7.50 x 10-2mol OH- 1 1000ml 1L = 1.39 x 10-3 mol OH- • Z : 35.00ml x 1L x 7.50 x 10-2mol OH- 1 1000ml 1L = 2.69 x 10-3 mol OH-

  11. 4. Determine the concentration of HA and A- ion X : [HA] = initial moles HA – moles OH- added volume of solution = 5.00 x 10-3 - 9.00 x 10-4 0.100 L = 4.10 x 10-2 Molarity Moles A- formed = moles OH- added [ A-] = 9.00 x 10-4 mol A- = 9.00 x 10-3Molarity 0.100L

  12. Y : [HA] = 5.00 x 10-3 – 1.39 x 10-3 0.1000L = 3.61 x 10-2 Molarity [A-] = 1.39 x 10-3 mol A- 0.1000L = 1.39 x 10-2 Molarity

  13. Z : [HA] = 5.00 x 10-3 - 2.63 x 10-3 0.10000L = 2.37 x 10-2 Molarity [A-] = 2.63 x 10-3 mol A- 0.10000L = 2.63 x 10-2 Molarity

  14. 5. Reciprocal of A- ion concentration • To show that the concentration is increasing, we need to flip the A- ion concentration. • X : 1/(9.00 x 10-3) = 1.11 x 10-2 M-1 Y : 1/(1.39 x 10-2) = 7.19 x 101 M-1 Z : 1/(2.63 x 10-2) = 3.80 x 101 M-1

  15. 6. Reciprocal of A- ion concentration against H3O+ ion concentration • If it is not reciprocal of A- ion concentration, then the slope (Ka) would be negative value.

  16. 7. Determine the slope of the line • Slope = ΔY = Δ[1/A-] ΔX = Δ[H30+] =1.11 x 102 – 3.80 x 101= 3.08 x 108 (3.16 x 10-7) – (7.94 x 10-8)

  17. 8. Determine the initial concentration of the acid, HA. = 5.00 x 10-3 mol HA = 5.00 x 10-2 M 0.10000L

  18. 9. Determine the Ka for the weak acid. 1 = Δ[H30+] = (H30+) · (A-) Slope Δ[1/A-] Ka = 1 = 1___________ HA x slope (5.00 x 10-2)·(3.08 x 101) = 6.49 x 10-8 M

  19. Conclusion • In this lab, I measured pH values of each solution and then determined the concentrations of HA and A- ion. I calculated the slope of the reciprocal of A- ion concentration on the ordinate against the H3O+ ion concentration and got the Ka for the weak acid, 6.49 x 10-8 M.

  20. Citation • http://answers.yahoo.com/question/index?qid=20090201045717AAjsY1g • http://wb7.itrademarket.com/pdimage/15/400615_cyberscanpc300.jpg

  21. Image citations • Slide #6 : http://www.micglobal.co.uk/images/beakers_14000BGYR.jpg • Slide # 7 : http://wb6.itrademarket.com/pdimage/15/400615_cyberscanpc300.jpg • Slide # 8 :http://www.flickr.com/photos/inthe_aeroplane/526874125/

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