I have spent the last several hours stuck on three questions.... here is one of
ID: 833091 • Letter: I
Question
I have spent the last several hours stuck on three questions.... here is one of them:
"Determine the pH change when 0.079 mol KOH is added to 1.00 L of a buffer solution that is 0.365 M in HCN and 0.394 M in CN-.
pH after addition ? pH before addition = pH change = _____"
I need to give 1 answer....
Here are the steps I have so far......
STEP 1: write out the balanced equation for the acid hydrolysis reaction.....is this correct?
STEP 2:
pH=pKa + log [CN-] / [HCN] = -log (4.0 x 10-10) + log (0.394 / 0.365) = ?1st mystery number? = 0.0921 (is that right?)
STEP 3:
mol HCN = 0.365 M x 1.00 L= 0.365 mol mol CN- = 0.394 M x 1.00 L = 0.394 mol
HCN(aq) + OH-(aq) ------> H2O(l) + CN-
Initial (mol) 0.365 0.079 0.394
Change (mol) -0.079 -0.079 +0.079
After rxn (mol) 0.286 0 0.473
STEP 4: [HCN]=0.286 mol HCN / 1.00 L = 0.286 M [CN-] = 0.473 mol CN- / 1.00 L = 0.473 M
pH=pKa + log( [CN-] / [HCN] )= -log (4.0 x 10-10) + log (0.473/0.286) = ?2nd mystery number? = 0.277 (right?)
change/delta pH= ?2nd mystery number? - ?1st mystery number? = 0.XXX = 0.XX 3rd mystery # (ok so I round the 0.XXX to 2 decimal places and this number shows, that the addition of 0.079 mol of KOH to the "buffer" increases the pH by 0.XX units.)
so is this right .... change pH= 0.277 - 0.0921 = 0.185 = 0.18 so adding 0.079 mol KOH to the buffer increases the pH by 0.18 units.........change pH= 0.18
PAUSE this is where I get more confused... is 0.XX aka 0.18 aka 3rd mystery number the answer that the stupid online evil computer homework software wants..... OR......is this the final answer below....
"If the same amount of KOH is added to 1.00 L of water, the pH increases by (4th mystery number "?.??") units, from a pH of 7.00 to a pH of (7.00 + 4th mystery number "?.??"). (how do I use 0.XX (aka 0.18) to get 4th mystery number "?.??")
so since I can only put one answer into the brain cell killing homework software, is the answer the "0.XX" (3rd mystery number that shows the units that the pH is increasing by), or is the final answer when you put it all together (the "?.??" 4th mystery number)?
Also if you have made it this far and would rather tell me on the phone my number is 404-354-5358 eastern time but since we are down to the last 2 weeks of the semester that means I am up all hours of the night trying to cram any last info my mind can absorb before finals so really call anytime or email me amanda.pass@hotmail.com if you don't want to respond on here.
Summary...
what are the 1st, 2nd, 3rd, and 4th mystery numbers?
are my 1st, 2nd, 3rd mystery numbers correct?
how do you use the 3rd mystery number to get the 4th mystery number (that aids in showing the total pH when added to 7.00)?
does the 3rd mystery number or 4th mystery number answer the computer's extragant time consuming question "pH change =" ? Because it seems like both the 3rd mystery number and 4th mystery number answer this, but I only get 1 shot at it and if I miss it I have to do this problem along with 29 other questions all over again.
Thanks in advance!
Amanda
also any tips on what helped you memorize this procedure and do it quickly are greatly appreciated.
side note: this is a homework excercise that we are allowed to receive outside help on that is due in a week.
Explanation / Answer
Hey Amanda! Let's solve and understand the question:
HCN + H2O ---> H3O+ + CN-
This is a buffer solution. Now adding CN- will not change the pH of the system as it will in case of 1 L water.
You have correctly solved the mystery number 1,2 and 3 ( No need to panic)