Experiment 8. Buffers I 15 March 2018

Transcription

1 Experiment 8. Buffers I 15 March 2018 Weak acid + conjugate weak base = buffer. Photo: Science in Hydroponics

2 Objec&ve: To prepare a buffer of a specific assigned ph from a weak acid and its conjugate weak base. This week and next week we will be making buffers! This could be fun and educaeonal. Overview: 1. Review making buffers and the Henderson Hasselbalch Buffer Equa&on 2. Choosing the best weak acid + conjugate base pair 3. The Math 4. Making the buffer 5. Procedure: What we do today 6. Your lab report 2

3 1. Review making buffers and the Henderson Hasselbalch Buffer Equa&on There are three recipes for making buffers. This week we will be using the first recipe. What could possibly go wrong? And next week we will use either Recipe 2 or 3. The 3 recipes for making a buffer: 1. Mix a weak acid with its conjugate weak base 2. React a weak acid with OH - as the limi&ng reagent 3. React a weak base with H 3 O + as the limi&ng reagent 3

4 1. Review making buffers and the Henderson Hasselbalch Buffer Equa&on n ph = pk a + log( wb ) n wa Info for IntroducFon So looking at the Henderson, we see that the ph depends on the number of moles of weak acid and moles of weak base and the pk a, of course. NoEce that if the moles of weak base equals the moles of weak acid, the log term equals zero and ph = pk a. Usually there is more of one than the other in order to get the desired ph. 4

5 2. Choosing the best weak acid + conjugate base pair n ph = pk a + log( wb ) n wa So the strategy is to pick an acid with a pk a similar to the desired ph. Then we can mix similar moles of weak acid and weak base. HA pk a = 4.75 HB pk a = 5.15 HC pk a = 6.05 HD pk a =

6 2. Choosing the best weak acid + conjugate base pair n ph = pk a + log( wb ) n wa HA pk a = 4.75 HB pk a = 5.15 So if the assigned ph was 6.00, for example, the best acid for the job would be HC and its conjugate base C -. HC pk a = 6.05 HD pk a = 7.45 And now we do a livle math. 6

7 3. The math n C - n HC ConEnuing on with this ph 6.00 example, we need to make a solueon in which the mole raeo of weak base to weak acid is = n wb n wa ph = pk a + log( ) n C = log( ) n = log( C - ) n HC n HC First solve for log(wb/wa). It s And then take the inverse log 10. So with log numbers, every digit to the right of the decimal is significant. The number has two significant figures. I know! Crazy. For sig figs, we get 0.89, but we should use through to the end of the calculaeons. Try this calculaeon yourself. See if you get

8 3. The math The instruceons said to make 50.0 ml of M buffer. n C - n HC = From n = MV, that means we want to make total moles of buffer (moles weak acid + moles of conjugate base) Wait. Two equaeons and two variables! n C - + n HC = moles Info for IntroducFon 8

9 3. The math We solve for n HC using a livle trick called math. n C - n HC = Two equaeons two variables Hmmm... n C - = 0.891n HC n C - + n HC = moles 0.891n HC + n HC = moles 1.891n HC = moles total n HC = moles HC 9

10 3. The math Then subtract. Boom. Done. n HC = moles HC n C - + n HC = moles So this is our plan to measure out this many moles of HC and C -. Dissolve them together in water. Voila! n HC = moles HC n C - = moles C - n total = moles 10

11 4. Making the buffer Easy peasy. Convert from moles to mass and mass out the weak acid and weak base and dissolve them in about 50 ml water! To review the process: 1. Use Henderson to solve for the raeo n wb /n wa. 2. Given n wb + n wa = mol, solve for n wb and n wa. 3. Convert the moles to masses and 4. Dissolve in deionized water! Of course each pair of us will have a different assigned ph, so let s start with picking the appropriate weak acid the one with the pk a that matches our assigned ph. Nothing going wrong so far 11

12 Weak Acid pk a MM Weak Base MM NaHSO g/mol Na 2 SO g/mol (sodium hydrogen sulfate or sodium bisulfate) NaH 2 PO 4 (sodium dihydrogen phosphate or sodium phosphate monobasic) (sodium sulfate) g/mol Na 2 HPO 4. 7 H 2 O g/mol (sodium hydrogen phosphate or sodium phosphate dibasic) NaHCO g/mol Na 2 CO g/mol (sodium hydrogen carbonate or sodium bicarbonate) 4. Making the buffer (sodium sulfate) These are our choices today for weak acids and conjugate bases. Start by picking the best one for your assigned ph. 12

13 4. Making the buffer A. Pick the best wa + wb from previous slide for your assigned buffer. B. Use Henderson to figure out mole raeo and moles of each C. Convert moles to masses. D. Make the buffer the exact amount of water doesn t maver, but should be around 50 ml. Info for IntroducFon 13

14 4. Making the buffer The buffer probably won t work out like the math it ll be close. We can make adjustments quite easily, however. See? Something went wrong. If the ph is too high, we can add drops of 1 M HCl(aq) unel it s perfect. Hmmm What should I do if the ph is too low? Info for IntroducFon 14

15 5. Procedure: What we do today 1 Wear your safety glasses. Dress for a mess. 2 Take Eme wrieng an introduceon in your own words before lab. 3 Record observaeons and details as carefully as possible. 4 Once you are ready to make your buffer, use the pocket balances at your staeon. They are good to the hundredths place. 5 When your buffer is ready, ask your TA to check it for accuracy. She/he will inieal the cover sheet if it s ok. Then clean up complete your on-line submission! 6 The cover sheet summarizes everything that you need to include with your report. 7 The ph probes are like goldfish they work bever if kept in water always. Stuff to do today. 15

16 6. Your Lab Report S&ck people inspired by xkcd cartoons by Randall Munroe ( 1 First, the cover page with TA ini&als. 2 Next, the trimmed copy pages from your lab notebook stapled together. 3 Staple all together. 4 On-line results due at the end of class today. Late submissions are not graded see the syllabus. 5 Turn in lab report today or before the start of class tomorrow. Late labs may not be graded see the syllabus. Happy St Pats Everyone! This is embarrassing 16