Recall that the pH scale typically runs from 0 to 14 (although it is possible to have a pH outside that range). The pink region is acidic (stomach acid, lemon juice), the purple neutral (pure water, saline solution) and the blue is basic (antacids, baking soda).
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
The
[H+] in solution is usually quite small, ∴
we usually express [H+] in terms of pH. A change in one unit of pH, changes the
concentration tenfold. You should recall
this equation from both grade 10 science and grade 11 chemistry:
pH = -log[H+]
ex. If [H+] = 1.0 x
10-7 M, what is the pH of the solution?
pH = -log [H+]
pH = -log(1.0 x 10-7)
pH = -log(1.0 x 10-7)
= 7.00*
ex. A sample of apple juice has a pH of
3.76. Calculate the hydrogen ion
concentration.
pH = -log [H+]
3.76 = -log[H+]
log[H+] = -3.76
(To remove the log, take the anti-log - it's the 10x button on your calculator - usually found as the second function on the log button.)
[H+] = 10-3.76
[H+] = 1.74 x
10-4 M*
*Tip: There are specific rules for rounding when using log. However, let's make life simpler and just use two decimal places, whether reporting a pH value or [H+] value in scientific notation.
pOH
We
can also calculate pOH:
pOH =
-log[OH-]
Starting
with the equation we were introduced to in the last lesson and incorporating
the pH and pOH equations, we can derive another equation that we be very
useful:
Starting with: [H+][OH-] =
1.0 x 10-14
Take log of both sides: log([H+][OH-]) = log(1.0 x 10-14)
log(xy) = logx + logy: log[H+] + log[OH-] = log(1.0 x 10-14)
Multiply each term by -1: -log[H+] + (-log[OH-]) = 14.00
Using equations for pH, pOH, gives: pH + pOH = 14.00
Measuring pH
pH
can be measured using a pH meter (pair of electrodes connected to a meter which
measures small voltages – the voltage varies with pH). A cruder method is with acid-base indicators,
like litmus or phenolphthalein.
Strong Acids
The
most common strong acids are HCl, HBr, HI, HNO3, HClO4, H2SO4.
Recall that a strong acid dissociates
completely into its ions (HA → H+ + A-).
ex. What is the pH of a 0.040 M solution
of perchloric acid?
HClO4(aq) → H+(aq) +
ClO4-(aq)
[H+]
= [ClO4-] = 0.040 M, due to the stoichiometry above
pH = -log[H+]
pH = -log(0.040)
pH = 1.40
Strong Bases
The most common strong bases are group 1 and group 2 metal hydroxides, like NaOH, KOH, Mg(OH)2 and Sr(OH)2.
Recall that a strong
base dissociates completely into its ions (MOH →
M+ + OH-).
ex. What is the pH of a 0.011 M solution
of Ca(OH)2?
Ca(OH)2(aq) → Ca2+(aq) +
2OH-(aq)
[OH-]
= 2(0.011 M) = 0.022 M, from the stoichometry above
pOH
= -log[OH-]
= -log(0.022)
= 1.66
pH + 1.66 = 14.00
pH = 12.34
Homework #6a-11
