lecture+10sf-1

# lecture+10sf-1 - pH of Strong Bases The most common...

This preview shows pages 1–4. Sign up to view the full content.

pH of Strong Bases The most common examples of strong bases are metallic hydroxides, most commonly hydroxides of group I and group II. Examples are NaOH, KOH, Ca(OH) 2 , Ba(OH) 2 . These compounds, when dissolved in water, dissociate completely into a metallic cation and the hydroxide ion. They are direct sources of hydroxide ion in aqueous solution. What is the pH of 0.10 M NaOH? Due to complete dissociation, [OH - ] = 0.10 M pOH = - log (0.10) = 1.00 pH = 13.00 What is the pH of 0.050 M NaOH? Due to complete dissociation, [OH - ] = 0.050 M pOH = - log (0.050) = 1.30 pH = 14 - 1.30 = 12.70 What mass of NaOH must be dissolved in a total of 300 mL of solution in order to produce a solution with pH 12.00? If pH = 12.00, then pOH = 2.00 Then [OH - ] = 10 -2.00 = 0.010 M 0.010 x 0.300 L x 40.0 = 0.12 g NaOH

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
A saturated Mg(OH) 2 solution has pH 10.52. Calculate the molar solubility of Mg(OH) 2 . pH = 10.52 pOH = 14.00 - 10.52 = 3.48 [OH - ] = 10 -3.48 = 3.3 x 10 -4 M Mg(OH) 2 (s) → Mg 2+ (aq) + 2OH - (aq) Every mol of Mg(OH) 2 dissolving produces 2 mol OH - . An equivalent statement: for every mol of OH - produced, mol of Mg(OH) 2 dissolves. If [OH - ] = 3.3 x 10 -4 M, then the mol Mg(OH) 2 dissolved per liter = 2 1 (3.3 x 10 -4 ) M = 1.7 x 10 -4 M. This is the molar solubility of Mg(OH) 2 . pH of Weak Bases Weak acids are substances which partially donate a proton to water. The strength of the weak acid is measured by its K a value. HCN + H 2 O H 3 O + + CN - K a = 6.2 x 10 -10 acid base vacid base Weak bases are substances which partially accept a proton from water. The strength of the weak base is measured by its K b value. The prototypical example of a weak base is ammonia, NH 3 . The lone pair on the N atom serves as a site to which H + can attract. An ammonia molecule attracts H + from water. Water functions as the acid, and when water donates an H + ion to water, hydroxide ions form: NH 3 + H 2 O NH 4 + + OH - K b = 1.8 x 10 -5 base acid acid base
3 . NH 3 + H 2 O NH 4 + +OH - K b = 1.8 x 10 -5 0.10 - x x x K b = ] NH [ ] OH ][ NH [ 3 4 - + = 1.8 x 10 -5 x 10 . 0 x 2 - = K b = 1.8 x 10 -5 Solving for x, making the usual approximation that x is small compared with 0.10, we get: x = [OH - ] = 1.34 x 10 -3 M pOH = -log[OH - ] = - log (1.34 x 10 -3 ) = 2.87 pH = 14.00 - pOH = 14.00 - 2.87 = 11.13 Percent ionization = 10 . 0 10 x 34 . 1 3 - x 100 = 1.34% Besides NH 3 , most other bases are organic molecules known as amines, which are close relatives of NH 3 . One or more carbon atoms replace the hydrogen atoms. Amines are weak bases, since the N atom still has a lone pair of electrons, which is a location which attracts an H + ion. The simplest amine is methylamine,

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### Page1 / 17

lecture+10sf-1 - pH of Strong Bases The most common...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online