Table of Contents

1. 4A-C Questions on logarithm notation
2. 4D Using logs - the pH scale.

4D Using logs - the pH scale.

Practice calculating pH from [H+] and vice versa.

 1 What is the pH of stomach acid which has a hydrogen ion concentration of 0.0125 M? pH = (to one decimal place) well doneyou need to calculate log10(0.0125)Your answer has been saved.

 2 Milk of magnesia has a hydrogen ion concentration of 3 x 10-10 M. Its pH is (to one decimal place) well doneyou need to take the log (to the base 10) of 3 x 10-10Your answer has been saved.

 3 Fresh milk has a hydrogen ion concentration of 4 x 10-7 M. What is its pH (to one decimal place)? well donetake the log of 4 x 10-7Your answer has been saved.

 4 Rainfall in Wisconsin varies from a pH of 4.4 in southeastern Wisconsin to nearly 5.0 in northwestern Wisconsin. This difference in pH of 0.6 units corresponds to what difference in hydrogen ion concentration? Thanks to the Wisconsin department of natural resources for the data. http://www.dnr.state.wi.us/org/water/fhp/lakes/under/acidity.htm The hydrogen ion concentration corresponding to pH 4.4 = μM (2 significant figures)the hydrogen ion concentration corresponding to pH 5.0 = μM (2 significant figures). The difference between these concentrations is μM. The concentration of hydrogen ions is times larger at pH 4.4 than pH 5.0. well doneyou need to antilog -4.4 (to get 40 μM) and -5.0 (to get 10 uM). To get the difference - you subtract (40 - 10 = 30 μM). To find out how much larger you can divide one by the other (40/10 = 4)Your answer has been saved.

 5 Anaesthetists have to ensure that a patient is breathing at an appropriate rate since the removal of CO2 from the blood is essential in maintaining blood pH within the normal range. In an experiment, an acute change in ventilation caused a drop in blood pH from 7.40 to 7.34. What was the corresponding difference in the concentration of hydrogen ions? pH 7.4 corresponds to [H+] = nM (to one decimal place) pH 7.34 corresponds to [H+] = nM (to one decimal place) difference in [H+] = nM (to one decimal place) well donecalculate 10-7.4 (= 3.98 x 10-8 M = 39.8 nM); similarly for 10-7.34 (= 45.7 nM). The difference is therefore 45.7 - 39.8 = 5.9 nM)Your answer has been saved.

 6 Calculate the pH of a 0.01 M solution of calcium hydroxide. Assume that calcium hydroxide dissociates completely. Remember that [H+] x [OH-] = 10-14 M2 pH = (to one decimal place) well doneremember that calcium is divalent, so calcium hydroxide is Ca(OH)2. Therefore [OH-] = 0.02 M. Therefore [H+] = 10-14 / 2 x 10-2 = 5 x 10-13. pH = 12.3Your answer has been saved.

 7 Calculate the pH of a solution of HCl prepared by diluting 5 ml of 1 M HCl into a total volume of 1 L. pH of resulting solution = well donethe resulting solution is 0.005 M HCl. The log of that is -2.3. Therefore the pH is 2.3.Your answer has been saved.

 8 What volume of 1 M HCl would be required to decrease the pH of one litre of a 10-3 M HCl solution by one pH unit? (Ignore the small change in volume) The volume of 1 M HCl required would be ml. well doneIn one litre of 10-3 M HCl there are 10-3 mol. A solution with a pH 1 unit lower would be 10-2 M HCl. This would contain 10-2 mol. Therefore we need to add 0.01 - 0.001 mol = 0.009 mol HCl. The volume required would be 0.009 mol / 1 mol.L-1 = 0.009 L = 9 mlYour answer has been saved.