Acids, bases, pH (1 Viewer)

[Aerlinn]

I am having trouble with several questions:
1. Solutions of the acid-base indicator methyl red contains an equilibrium mixture of the red-coloured acid form of the indicator and a yellow-coloured conjugate form of the indicator and a yellow-coloured conjugate base form. The equilibrium can be represented by:
HMr(aq) <---> H+(aq) + Mr-(aq)
red yellow
The equilibrium constant for the reaction is 10^-5M at 25C.
a. Calculate the value of the ratio [Mr-]/[HMr] when the indicator is dissolved in a solution of pH 2. (The answer to this is 10^ -3, but I don't understand how it was calculated...)
b. Predict the colour of the indicator in this solution... (I don't understand how to find the answer...)
2. A buffer made from sodium hydrogen carbonate (NaHCO3) and carbonic acid (H2CO3) is sometimes used in swimming pools. Using this buffer as an example, explain how buffers operate.
3. The following equilibia are involved in the transport of carbon dioxide throughout the body:
CO2(g) <---> CO2(aq)
CO2(aq) + H2O(l) <---> H2CO3(aq) <---> H+(aq) + HCO3-(aq)
Buffers in the blood prevent these reactions from causing large changes in pH.
a. Use Le Chatelier's Principle to explain what would other wise happen to the pH in i) the lungs, and ii) the tissues
b. Hyperventilation results from rapid breathing.
i) When might a person experience hyperventilation?
ii) Use the above equations to explain what effect hyperventilation has on blood pH.
iii) During cardiac arrest the heart stops beating, but other cellular functions continue. Doctors sometimes quickly inject a sodium hydrogen carbonate solution directly into the heart muslce before restarting the heart. Why?
4. In pure water at 25C, the concetnration of both H30+ and OH- ions is 10^-7 due to the reaction described by the equation:
2H20(l) <---> H30+(aq) + OH-(aq)
The forward reaction is endothermic. Suppose some pure water is heated. What happens to the pH and why?
5. A 0.100M solution of hypiodous acid (HOI) has a pH of 5.8
The ionisation equation: HOI(aq) + H20(l) <----> OI- (aq) + H3O+(aq)
and K= [H3O+][OI-]/ [HOI]
But it says: 'Calculate [H3O+] in the solution'... how do we do that?


Thanks in advance ^^
:wave:

 

[xiao1985]

q1) a) pH = 2 : -log [H+] = 2.... [H+] = 0.01
k = 10^-5 mol/L = [H+][Mr-]/[HMr]
plug everything in, you should get your answer

b)ratio is extremely small... meaning conjugate base <<<<< conjugate acid meaning orange <<<<<< red... hence the solution will be red in colour...

2) NaHCO3(aq) <----> Na(aq) + HCO3-(aq) <---> H+(aq) + CO3 2-(aq) + Na+ (aq)
What happen if you add H+ or add OH- (thereby, removing H+) to the equilibrium?

3) a) in lung, [CO2] drops, in tissues, [CO2] rises... argue the rest =)
b) i) is that really a chemistry question? well, if you really want to: Hyperventilation can also occur as a consequence of various lung diseases, head injury, coffee abuse (from wiki)
ii) [CO2] drops sharply... what happens now?
iii) heart stop beating = acidity in other tissues build up... once start beating, the excessive acid will move to heart = bad for heart.... sodium bicarbonate = buffer = not too high concentration to pH

4) endothermic = + heat on reactant (LHS) ... increase temp =???

5) errr... pH = -log [H3O+]

 

[Aerlinn]

Hehh, I know, some of those were dumb questions >.<

2. I got these two (not sure it's right):
H2CO3(aq) + OH-(aq) <---> HCO3-(aq) + H2O(l)
HCO3-(aq) + H+(aq) <---> H2CO3(AQ) + h2O(l)
I'm not entirely sure how you got the equations *puzzled*
3. Yeah, it sounds more like a biology question...
iii) Can't say I follow you on this one... would you be able to explain slightly?

 

[wrxsti]

wtf? wat kinda question is this.........

 

[xiao1985]

2) the equation... well it's a bicarbonate ion dissociation (act as a acid) / bicarbonate ion ionises (act as a base) ... just the 2nd step in the dissociation of hydrogen carbonate... you kinda need to know this in one of the syllabus dot point... that's how i know this...

yes... you are absolutely right.... notice in the solution of NaHCO3, there are both HCO3 - and H2CO3 present...

if base is added, then first reaction will happen, consuming some OH- (we ADD OH-, but the buffer reacts such that OH- is removed)

if acid is addd, then 2nd reaction will happen, consuming some H+ (we ADD H+, but the buffer reacts such that H+ is removed)

so in either case, which ever reagent (H+ or OH-) we add, the system will shift in such a way such that the effect will be minimal... hence we call this a buffer (minimise the pH chnage in a system)

 

[Aerlinn]

The buffer stuff makes sense

notice in the solution of NaHCO3, there are both HCO3 - and H2CO3 present...

 

[xiao1985]

you start with NaHCO3, dissolve in water, the Na+ will stay Na+ (it's a good boy), the HCO3- will react with water in 2 ways:

HCO3- + H2O <----> H2CO3 + OH-

the other way happens in very small amount, so we don't have to worry too much:

HCO3- + H2O <-----> H3O+ +CO3 2-

therefore, in a solution of NaHCO3, you'd expect to find both HCO3 - and H2CO3