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

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1 Chemistry Hourly II review notes and questions to check your understanding. N. Kerner

2 Chem. 125/126 Review E4F07 Hourly II page 1/5 E4 Acids, Bases and Salts 1. Part 1: Acid-base strength, ph and % ionization The lower (higher) the ph of equimolar acids (bases) the stronger the acid (base) ph = -log [H 3 O + ]; [H 3 O + ] [OH - ] = 1 x ; acids = ph < 7; bases = ph >7 Strong or Weak. o Strong (weak) acids/bases ionize 100% (<100%);% ionization can be determined from ph % ionization acid = [H 3 O + ]/[acid] i x 100% ; % ionization base = [OH - ]/[base] i x 100% % ionization can be calculated as : [H+] / [HA] *100 How do you get [H+]? Yes it s given to you in ph or [H+] = 10 -ph Therefore, % ionization = 10 -ph / [HA] *100 Note that in order to know whether an acid is strong or weak, we need two pieces of information. One is the concentration of the acid, [HA] and the other its ph. To avoid confusion, just assume we would have a strong acid. HA H + + A - For the case of the strong acid, [HA] is equal to [H + ] because the acid is 100 % ionized. Example: 0.01 M of HA. Assuming that it s strong, [HA] = [H+] =0.01 and ph= -log (0.01) = 2. Now we can check a given ph value. If it was 2, the acid is strong. What about ph = ? It is WEAK. The point is that a value other than 2 tells us it s weak. In conclusion, at any concentration, we can calculate ph by assuming it s strong. Then compare the calculated value with the given one. If those are the same, it s strong. Otherwise it s weak. 2. Part 1: Acid-base props of compounds with element & O & H(XOaHb compounds): Are related to the position of X in the periodic table X = metal, compound is base (ex. NaOH); X = nonmetal, compound is acid (ex. HNO 3 ) Are related to the electronegativity of X The higher (or lower) the electronegativity of X the more acidic (more basic) the compound. Examples: 1. H 3 PO 3 (electronegativity P = 2.1) stronger acid than H 3 BO 3 (electronegativity B = 2.0) and thus ph H 3 PO 3 lower than ph H 3 BO 3 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. Part 2: Conjugate Acids and Bases There is a reciprocal relationship between acid/base and conjugate base/acid strength. You can make conjugate base by removing H + and conjugate acid by adding H + : + H + NH 3 NH H + Examples: 1. ph tests indicate HCN is a weak acid and CN - is a weak conjugate base Weak acids have weak conjugated bases and weak bases have weak conjugate acids. 2. ph HCN higher than ph HCl for equimolar solutions and therefore Acid Strength: HCl > HCN

3 Chem. 125/126 Review E4F07 Hourly II page 2/5 can predict Base strength of conjugates: CN - > Cl - 3. 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. Part 3: Acid-Base Neutralization and Indicators The salt product of acid-base neutralization is not necessarily neutral. (e.g. NaCl from HCl + NaOH is neutral; NaC 2 H 3 O 2 from HC 2 H 3 O 2 + NaOH is basic) Acidic/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 + NH 3 = NH 4 Cl (Cl - is spectator; NH 4 + is weak conjugate acid) You should be able to know what salts were made from, when you see the salts. For example, NH 4 NO 3 is made from HNO 3 and NH 3 and CH 3 COONa is from CH 3 COOH and NaOH. By checking whether acids and bases are strong or weak, you can estimate acidity of salts. For instance, NaNO 3 (neutral salt) has a higher ph than NH 4 NO 3 (acidic salt). The indicator end point ph should = the equivalence point ph of the titration products Note: the ph of salt and water products = equivalence point ph. For example, indicator should have end point ph 7 if salt products are 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) For example, the 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) Equivalent Wt (experimental) = g sample OR Equivalent Wt.(theoretical) = Molecular wt mol of titrant # ionizable H

4 Chem. 125/126 Review E4F07 Hourly II page 3/5 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) 2. Below are observed ph readings for a group of acids: A. Based on the ph data, rank the acid strength: 0.10 M acid ph Acid Strength HCN 4.89 HCl 1.00 > > HC7H5O (try Dec 04, 1A-C, April 05, 1B,1D; Dec.05, 1A. 3A; April 06, 1D, 2A; Dec.06, 2A, April 07, 1A, 3A) 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 =

5 Chem. 125/126 Review E4F07 Hourly II page 4/5 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 04, 1D-F; April 05 2(1,2,3,4); Dec.05, 1C, 3B; April 06, 1A. 2B; Dec.06, 2B; April 07, 3B-C) 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 B. Ethyl red C. Litmus D. Thymol blue M Acid acid ph Salt /acidic,basic,neutral? Indicator to use (A, B, C, or D) CH 3 CH 2 COOH 2.9 HNO You titrate g of an acid unknown with 0.10 M NaOH and ml of base is needed to reach the end point. Your calculated equivalent weight for the acid is The titration was performed using phenolpthalein (ph color change interval = ). You repeat the above titration (i.e. you titrate 0.193g of the unknown acid) but use methyl orange indicator (ph color change interval = ). 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

6 Chem. 125/126 Review E4F07 Hourly II page 5/5 B. Indicate (Circle) the effect of the result on the calculated equivalent weight of the acid more than equal to less than (now try Dec 04, 1B. April 05, 4D; Dec.05, 4A; April 06, 2C; Dec.06, 3 B-D; April 07, 2C) Part 5. Identification of an Unknown Acid 6. You are to identify an unknown that is one of the following compounds: mp( o C) Name Formula MW 17 acetic acid CH3COOH ,3-dimethylglutaric acid HOOCCH 2 C(CH 3 ) 2 CH 2 COOH acrylic acid H 2 C=CHCOOH propanoic acid CH3CH2COOH maleic acid HOOCCH=CHCOOH cyclohexane carboxylic acid C6H11COOH pentanoic acid CH3CH2CH2CH2COOH chloro, 1,2 benzene dicarboxylic acid ClC6H4 (COOH) pentenoic acid CH3=CHCH2CH2COOH 100 You titrate g of the unknown with 0.100M NaOH ml of base is needed to reach the end point. You now titrate g of the unknown with 0.100M NaOH and 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 (25 o C). 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 04, 2A, April 05, 4; Dec.05, 4B; April 06, 3; Dec.06, 3A; April 07, 2)