Advanced Chemistry Practice Problems

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Buddy Woods
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1 Advaced hemistry Practice Problems Aqueous Equilibria: Buffer Actio 1. Questio: A small amout of HBr is added to a buffer which is 0.10 M HHO ad 0.15 M NaHO. What species i the buffer will eutralize the added acid? Write the eutralizatio reactio. Aswer: The strog acid (HBr) will be eutralized by the compoet of the buffer - (amely, HO of the salt) accordig to the reactio: H 3 O + (aq) + H O - (aq) HH O(aq) + H O(l) Note: The strog acid is represeted as H 3 O + i the reactio because Hl will completely react with water to produce this io ad Br - (a spectator io).. Questio: A small amout of a NaOH is added to a buffer which is 0.10 M HHO ad 0.15 M NaHO. What species i the buffer will eutralize the added? Write the eutralizatio reactio. Aswer: The strog (NaOH) will be eutralized by the acid compoet of the buffer, that is, HH O, accordig to the reactio: OH - (aq) + HH O(aq) H O - (aq) + H O(l) Note: The strog is represeted as OH - i the reactio becuase NaOH will completly ioize i water to produce OH - ios ad Na + ios (the Na + is a spectator io ad therefore ot writte.)

2 Advaced hemistry Practice Problems 3. Questio: alculate the ph of a solutio if 1.50 m of 0.10 M HBr is added to m of a buffer which is 0.10 M HHO ad 0.15 M NaHO. K a (HHO ) = Aswer: To calculate the ph of this buffer solutio, we will first setup a I table that represets the oe-way reactio that occurs whe the strog acid is eutralized by the. This will be a table associated with the reactio give i Questio 1 above. moles H 3 O + (aq) + H O - (aq) HH O(aq) + H O(l) I To obtai the I lie, moles of each substace eeds to be calculated. Moles of H 3 O + comes from the strog acid: mol H O 0.10 mol mol or mol H O 3 Moles of the other substaces comes from the iformatoi about the buffer: mol H O mol mol H O mol HH O 0.10 mol HH O moles H 3 O + (aq) + H O - (aq) HH O(aq) + H O(l) I

3 Advaced hemistry Practice Problems The table is the completed by kowig that the limitig reactat will be completely cosumed: moles H 3 O + (aq) + H O - (aq) HH O(aq) + H O(l) I Now, we fiish the problem by realizig that what remais (as depicted i the lie) is still a buffer ad the Hederso-Hasselbalch equatio will apply. ph pk a log b as e b as e ph - log( ) log mol mol ph ph 3.91

4 Advaced hemistry Practice Problems 4. Questio: alculate the ph of a solutio if.00 m of 0.15 M NaOH is added to to m of a buffer which is 0.10 M HHO ad 0.15 M NaHO. K a (HHO ) = Aswer: To calculate the ph of this buffer solutio, we will first do a I table that represets the oe-way reactio that occurs whe the strog is eutralized. This will be a table associated with the reactio give i Questio above. moles OH - (aq) + HH O(aq) H O - (aq) + H O(l) I To obtai the I lie, moles of each substace eeds to be calculated. Moles of OH comes from the strog : - mol -4 - mol OH mol or mol OH Moles of the other substaces comes from the iformatoi about the buffer: - mol mol H O mol H O mol mol HH O HH O moles OH - (aq) + HH O(aq) H O - (aq) + H O(l) I The table is the completed by kowig that the limitig reactat will be completly cosumed: moles OH - (aq) + HH O(aq) H O - (aq) + H O(l) I Now, we fiish the problem by realizig that what remais (as depicted i the lie) is still a buffer ad the Hederso-Hasselbalch equatio will apply.

5 Advaced hemistry Practice Problems ph pka log -4 ph - log( ) log mol mol ph ph 3.94

6 Advaced hemistry Practice Problems Aqueous Equilibria: Buffer: Preparatio ad apacity 1. Questio: rom the list below, choose a acid ad its cojugate or a ad its cojugate acid which would be best to use to prepare of a buffer with ph = 3.70 Acid K a Base K b Acetic acid, H H 3O Ammoia, NH ormic acid, HHO Methylamie, H 3NH Aswer: To prepare a buffer of a specific ph, the first thig which must be decided is the acid/ cojugate pair which would make a good buffer for this solutio. hoose a combiatio which has a pk a close to the ph desired. A acidic buffer should be d upo a acid ad its cojugate salt ad ot a ad its cojugate acid sald. or acetic acid: pk a = -log( ) = 4.74 or formic acid: pk a = -log( ) = 3.77 The formic acid will be the best choice. Extra istructio to you: The ammoia ad the methylamie whould be good choices for buffers with high ph values. Determie the pk a o the cojugate acids of the give s as follows: or ammoia: log pk a or methylamie: log pk a

7 Advaced hemistry Practice Problems. Questio: Determie the umber of grams of sodium acetate that must be added to m of 0.10 M acetic acid, H H 3O, to prepare a buffer with ph = Aswer: Use the Hederso Hasselbalch equatio to solve for the moles of eeded, give that the moles of acid is 0.10 mol/ = acid: ph pka log acid 4.55 log( ) log log log 0.19 log log 0.010mo l ) mol This gives the moles of H 3O - eeded. This will be the same as the moles of Na H 3O (the salt) eeded. overt moles to grams: 8.04 g NaH3O mol NaH3O 0.53 g NaH3O 1mol Na H O 3

Solutions to Equilibrium Practice Problems

Solutions to Equilibrium Practice Problems Solutios to Equilibrium Practice Problems Chem09 Fial Booklet Problem 1. Solutio: PO 4 10 eq The expressio for K 3 5 P O 4 eq eq PO 4 10 iit 1 M I (a) Q 1 3, the reactio proceeds to the right. 5 5 P O