Chemistry 125/126 Hourly II review notes and questions to check your understanding.

N. Kerner Chem. 125/126 Review E4F08 Hourly II page 1/5

E4 Acids, Bases and Salts 1. Acid-base strength, pH and % ionization(Part 1A) - The lower (higher) the pH of equimolar acids (bases) the stronger the acid (base) pH = -log [H3O+]; [H3O+] [OH-] = 1 x 10-14; acids = pH < 7; bases = pH >7 - Strong (weak) acids/bases ionize 100% (<100%);% ionization can be determined from pH + - % ionization acid = [H3O ]/[acid]i x 100%; % ionization base = [OH ]/[base]i x 100% 2. Acid-base props of compounds with element & O & H(XOaHb compounds, part 1B): - are related to the position of X in the periodic table X = metal, compound is base (ex. NaOH); X = nonmetal, compound is acid (ex. HNO3) - are related to the electronegativity of X The higher (or lower) the electronegativity of X the more acidic (more basic) the compound. Examples: 1. H3PO3 (electronegativity P = 2.1) stronger acid than H3BO3 (electronegativity B = 2.0) and thus pH H3PO3 lower than pH H3BO3 for equimolar solutions. 2. NaOH stronger base (electronegativity Na = 0.9 ) than Al(OH)3 (electronegativity Al = 1.5) and thus pH NaOH higher than pH Al(OH)3 for equimolar solutions. 3. Conjugate Acids and Bases (Part 2) - Strength of acids (bases) and their conjugate bases (acids) are inverse. Observed: pH HCN higher than pH HCl for equimolar solutions and therefore Acid Strength: HCl > HCN ∴ can predict Base strength of conjugates: CN- > Cl- - Conjugate strength is predictable given the strength of its conjugate pair Examples: 1. pH tests indicate HCN is a weak acid and ∴ CN- is a weak conjugate base 2. pH tests indicate HCl is a strong acid and ∴ Cl- is a very weak conjugate base note: Cl- ion is such a weak base that it is, in effect, a spectator ion.

4. Acid-Base Neutralization and Indicators (Part 3) - the salt product of acid-base neutralization is not necessarily neutral.

(e.g. NaCl from HCl + NaOH is neutral; NaC2H3O2 from HC2H3O2 + NaOH is basic) - Properties of salt can be predicted from strengths of reacting acid and base.

Examples: 1. Strong acid + strong base = neutral salt (conjugate base & acid product are spectators) HCl + NaOH = NaCl (Cl- and Na+ are spectator base & acid ions) 2. Weak acid + strong base = basic salt (weak conjugate base; conjugate acid is spectator) HCN + NaOH = NaCN (CN- = weak base; Na+ = spectator ion) 3. Strong acid + weak base = acidic salt (conjugate base is spectator; conjugate acid is weak) HCl + NH3 = NH4Cl (Cl- is spectator; NH4+ is weak conjugate acid) Chem. 125/126 Review E4F08 Hourly II page 2/5

- Indicator choice effects titration results. - the chosen indicator end point pH should = the equivalence point pH. note: the pH of salt and water products = equivalence point pH. For example, indicator should have end point pH ≅ 7 if salt product is neutral (e.g.NaCl). Indicator end point pH should ≅pH 8-9 if salt product is a weak base - The higher (lower) the indicator pH end point, the more (less) titrant (e.g. NaOH) will be required to reach the end point (indicator color change) Ex.Class data shows greater volume (mL) NaOH used titrating acetic acid with phenolpthalein (end point pH 9.1) than bromophenol blue indicator (end point pH 3.8)

5. Equivalent Weight Titrations (Parts 4-5) Experimental Equivalent Wt = g sample OR Theoretical Equivalent Wt. = Molecular wt mol of titrant # ionizable H

EXPERIMENT 4 sample Questions Part 1 Structure and Acid-Base Properties 1. A. Based on your knowledge of chemical periodicity, predict the relative acid strength of H2SO4, H3PO4, HClO4, and Mg(OH)2.

weakest acid strongest acid ______< ______< ______<______

B. The electronegativity of phosphorous (P) is greater than the electronegativity of arsenic (As). Predict the comparative pH of 0.10M H3PO4 and 0.10M H3AsO4.

Indicate (circle) if the pH of 0.10M H3AsO4 is

Less than equal to greater than pH H3PO4 (pH 1.5) Chem. 125/126 Review E4F08 Hourly II page 3/5

2. Below are observed pH readings for a group of acids: A. Based on the pH data, rank the acid strength: 0.10M acid pH Conjugate Base Acid Strength

HCN 4.89

HCl 1.00 ------> ------> ------

HC7H5O2 2.96

(try Dec.05, 1A. 3A; April’06, 1B, 2A; Dec.06, 2A; April’07, 1A, 3A; Dec.07, 1 (1 & 2), 2A); April’08, 2D, 3

Part 2. Conjugate Acid-Base Pairs 2. B. Complete the above table (column 3) by filling in the conjugate base formula of each acid. C. Based on the above pH data which 0.10 M salt solution has the highest pH: NaCl, NaC7H5O2, or NaCN ?

The salt with the highest pH =

3. Below are observed pH readings for 0.10M solutions of sodium salts: sodium acetate = pH 7.4; sodium benzoate = pH 8.4; sodium lactate = pH 8.0

Predict the comparative acid strength of benzoic, lactic, and acetic acids. strongest acid weakest acid ______> ______> ______

(try Dec.05, 1C, 3B; April’06, 1A. 2B; Dec.06, 2B; April’07, 3B, 3C; Dec.’07, 2B; April’08, 3C1) Chem. 125/126 Review E4F08 Hourly II page 4/5

Parts 3-4. Acid-Base Neutralization & Indicator Color Changes 4. You perform several titrations (listed below) using 0.01M NaOH. For each titration: − give the formula of the salt that will be formed at neutralization - indicate if the salt and water product will be acidic, basic, or neutral − choose the best of four available indicators for each titration. Available Indicators pH range where indicator changes A. Metanil yellow 1.2-2.4 B. Ethyl red 4.0-5.8 C. Litmus 6.0-8.0 D. Thymol blue 8.0-9.6

0.01M Acid acid pH Salt /acidic,basic,neutral? Indicator to use (A, B, C, or D)

CH3CH2COOH 2.9

HNO3 2.0

5. You titrate 0.193 g of an acid unknown with 0.10 M NaOH and 32.99 mL of base is needed to reach the end point. Your calculated equivalent weight for the acid is 58.50. The titration was performed using phenolpthalein (pH color change interval = 8.2-10.0).

You repeat the above titration (i.e. you titrate 0.193 g of the unknown acid) but use methyl orange indicator (pH color change interval = 3.2-4.4) . Determine the effect, if any, on: A. The volume of 0.10M NaOH required to reach the end point. B. The calculated equivalent weight of the acid. A. Indicate (Circle) the volume of 0.10M NaOH used at the end point.

more than 32.99mL Equal to 32.99mL less than 32.99mL

B. Indicate (Circle) the effect of the result on the calculated equivalent weight of the acid

more than 58.50 equal to 58.50 less than 58.50 Chem. 125/126 Review E4F08 Hourly II page 5/5

Part 5. Identification of an Unknown Acid 6. You are to identify an unknown that is one of the following compounds:

mp(oC) Name Formula MW 17 acetic acid CH3COOH 60 103 3,3-dimethylglutaric HOOCCH2C(CH3)2CH2COOH 160 acid 13 acrylic acid H2C=CHCOOH 72 -23 propanoic acid CH3CH2COOH 74 139-140 maleic acid HOOCCH=CHCOOH 116 31-2 cyclohexane C6H11COOH 128 carboxylic acid -33.8 pentanoic acid CH3CH2CH2CH2COOH 102 186-7 3-chloro, 1,2 benzene ClC6H4 (COOH)2 200 dicarboxylic acid -22.5 4-pentenoic acid CH3=CHCH2CH2COOH 100 You titrate 0.374 g of the unknown with 0.100M NaOH. 37.03 mL of base is needed to reach the end point. You now titrate 0.484 g of the unknown with 0.100M NaOH and 47.40 mL is required to reach the end point. What is the unknown's equivalent weight?

the equiv. weight =

Based solely on the calculated equivalent weight, list all compounds that may be the unknown:

The unknown compound is a solid at room temperature (25oC). Based on this fact and the calculated equivalent weight, identify the unknown. Based on all the above tests and observations, the unknown most likely is:

(now try Dec.05, 4A; April ’06, 2C; Dec.06, 3 B-D; April’07, 2; Dec.07, 3; April’08, 4)