pH question (1 Viewer)

[RachelGreen]

Can anyone show working out for (a), (b) and (c)

I know the pH change for (d) is 0.01 to 1.01

 

[RachelGreen]

Thanks in advance!!

 

[Jippyjitsu1]

It's a multiple choice answer, it asking which of these would cause the greatest change the CaCo3 is basic NaOH is basic and HCl is acidic, adding the HCl will do nothing, it might increase a little but not much, its between a,b with NaOH being the most basic or Alkaline meaning that b should be the answer. ( I think CaCO3 is marble...)

 

[leehuan]

c) is the easiest case. You're mixing two samples with the same pH. That won't change the pH at all.
pH = -log_10(0.1) = 1

d) is essentially a dilution. A dilution tenfold trivially increases the pH by one.
pH = 2

Proof:
C1V1=C2V2
0.1molL^-1 * 100mL = C2 * 1000mL
0.01molL^-1 = C2
Conc. HCl = Conc. H3O+
pH = -log(0.01) = 2

b) is a neutralisation. HCl + NaOH -> NaCl + H2O
1:1 molar ratio
n(HCl) = 0.1 * 0.1 = 0.01mol
n(NaOH) = 0.01 * 0.1 = 0.001mol
Hence NaOH is the limiting reagent
n(HCl) after reaction = 0.01-0.001 = 0.009mol
Volume after reaction = 110mL
Hence [H3O+] = [HCl] = 0.009/(110*10^-3) = 0.081818181818181..
pH = -log_10[H3O+] = 1.08

a) is the more dodgy one. Also a neutralisation. 2 HCl + CaCO3 -> CO2 + H2O + CaCl2
The process is similar to b) - use the concept of a limiting reagent. But I'll leave this one alone.

Of course, to calculate the moles of CaCO3 you need n=m/MM

 

[leehuan]

It's a multiple choice answer, it asking which of these would cause the greatest change the CaCo3 is basic NaOH is basic and HCl is acidic, adding the HCl will do nothing, it might increase a little but not much, its between a,b with NaOH being the most basic or Alkaline meaning that b should be the answer. ( I think CaCO3 is marble...)

CaCO3 is limestone so yes virtually marble.

But whilst I do doubt the authenticity of my calculations, if they are correct then you would be surprised to find that because we add such a limited amount of NaOH we cause little harm to the system's pH..

 

[RachelGreen]

I have my half yearlies on Thursday. Do you have any tips or have anything to tell me to revise over specifically?

 

[porcupinetree]

I have my half yearlies on Thursday. Do you have any tips or have anything to tell me to revise over specifically?

Revise everything and anything in the first two topics in the syllabus

 

[leehuan]

^Well tbh revise anything that has been taught. Some schools will drop out sections in topics if not taught.

But try looking for some half yearly papers. I'm not sure if dan has any

 

[RachelGreen]

Hey can you also help me with this difficult POM question, I know this is the wrong thread to post this in but who cares.

A scientist placed a rock on top of a few sheets of newspaper, under which was an envelope containing photographic film. After only 5 minutes he removed the photographic film and found that an image in the shape of the rocket had developed on the film. After 28 days, he demonstrated this phenomenon to his colleagues with the same rock, but this time it took 20 minutes for the image to develop to the same intensity.

This table shows properties of some radioactive isotopes:
Name of isotope/Radiation emitted/Approximate half-life

Sodium-24/beta,gamma/15 hours
Bismuth-210/beta/5 days
Iodine-131/beta/7 days
Phosphorous-32/beta/14 days
Radium-225/alpha/14 days
Thorium-227/alpha/28 days
Protactinium-234/beta/28days

Use information about the scientist's experiment and that in the table above to deduce which of these radioactive isotopes the rock may have contained. Explain your choice. [2]

I'm able to explain why it is phosphorous-32 in terms of the radiation. The radiation has to be gamma or beta, to penetrate through the objects, but I can't get the second mark to explain in terms of short half life


And this Jame's Ruse MC question got me stuck, Idk why I'm having so much problems with pH questions

Thanks to whoever helps me with the exam questions!

 

[leehuan]

I can't articulate it carefully but note that initially the decay occurred in 5min

But then 28 days later the decay occurred in 20min

Incidentally, the half life of the phosphorus is 14 days
You can just think about it logically
Initially - decays over 5min
14 days later - decays over 10 min
28 days later - decays over 20 min
42 days later - decays over 40 min

etc.

 

[leehuan]

I'll just do the method for the pH question

1. write out equation
2. determine molar ratio
3. determine initial moles of Ba(OH)2 and HCl
4. identify limiting reagent
5. determine quantity (moles) of excess reagent left
6. find concentration of excess reagent (note: new volume is 60.0mL)
7. apply dilutions formula C1V1=C2V2
8. calculate pH

Possible step 7.5 - If Ba(OH)2 is the excess reagent, you may need Kw = 1.0*10^-14 = [H3O+]*[OH-]
Or use pH + pOH = 14

 

[RachelGreen]

Oh cool, thanks I got it!

 

[RachelGreen]

Wait, hold on what's step 7.5 "Kw = 1.0*10^-14 = [H3O+]*[OH-]"

 

[leehuan]

Self ionisation of water