help with questions

[Rawza]

Soz Guys, that was for a question agez ago, lol

[Rosie]

Q. Carbon disulfide gas (CS2), which is used in the manufacture of rayon, can be made using an endothermic gas-phase reaction between sulfur trioxide gas (SO3) and carbon dioxide. Oxygen gas is also produced in the reaction.
An equilibrium mixture of these gases was made by mixing sulfur trioxide and carbon dioxide. It consisted of 0.028 mol of CS2, 0.022 mol of SO3 and 0.014 mol of CO2 in a 20L vessel. Calculate the value of the equilibrium constant at that temperature.


acid-base question
Q. What mass of hydrogen chloride gas must be dissolved in 1L of hydrochloric acid to change the pH from 2 to 1, if it is assumed that no change in volume occurs?

i'm really confused by this question 

[Mao]

hence,


dissolving hydrogen chloride gas:


which then
(this is really an equilibrium, but because the acid almost completely ionises, it is easier to assume it does completely ionise)

originally, , to increase it to in a 1L container, we need , now use to find the mass of HCl required,

[Rosie]

Q. Which of the following mixtures would have a pH of 2?
a. 100mL of HCl with pH = 3 + 10mL of HCl with pH = 1
b. 100mL of HCl with pH = 3 + 100mL of HCl with pH = 1
c. 100mL of NaOH with pH = 13 + 100mL of HCl with pH = 1
d. 100mL of HCl with pH = 1 + 100mL of water


Q. If 100mL of 0.05M solution of nutric acid is diluted to 500mL, then the pH of the resultant solution would be approximately
a. 2
b. 3
c. 4
d. 5


thanks

[iamdan08]

For question 1 the answer is A. The working out is as follows:

First we have to calculate the total mol of the HCl acid:

mol of 100ml HCl with ph=3                mol of 10ml HCl with ph=1                  (remembering that the pH of a strong acid is
                                     
                           
                                   


You then add the total mol and total volume:


       

Calculate concentration:

   
   


Then calculate pH:
Since
         
         

THis question is essentially a process of elimination.

[iamdan08]

For the second question, you use the dilution formula (pretty sure thats what its called):





so therefore

Chuck it into

And you get a pH=2



So the answer is A.

[Rosie]

thanks for those detailed answers

Q. The pH of a solution is changed from 2 to 5. This means that the concentration of
a. hydroxide ion has decreased by a factor of 3
b. hydroxide ion has increased by a factor of 1000
c. hydrogen ion has decreased by a factor of 3
d. hydrogen ion has decreased by a factor of 1000

[Collin Li]

Factor of 1000, because that's a movement from to , which is a factor.

It represents a dilution of acid, so the hydrogen ion concentration has decreased by a factor of 1000. Also, the hydroxide ion has increased by a factor of 1000, as they are related via the equation

Option B and D are correct, unless you have miscopied the question.

[dusty_girl1144]

thanks for those detailed answers

Q. The pH of a solution is changed from 2 to 5. This means that the concentration of
a. hydroxide ion has decreased by a factor of 3
b. hydroxide ion has increased by a factor of 1000
c. hydrogen ion has decreased by a factor of 3
d. hydrogen ion has decreased by a factor of 1000

im assuming you have the chemistry check points 08 =D i got stuck with which multi choice to answer ^^ the answer they have given  is B. but i dont get why they put 2 correct answers  .... what would you do in an exam situation?

[cara.mel]

In an exam situation if they have 2 answers that are true or can be interpreted to be true, they will usually accept both

[shinny]

You'd just pick one of them. Although this is just my assumption, once VCAA realises, I'm pretty sure the mc scanny thingy will just register either answer as correct, but having BOTH might not necessarily turn out so well. Don't bother questioning the supervisors either...they're pretty clueless as to what's on the exam paper.

[dusty_girl1144]

isant that a bit stupid tho? :s

[cara.mel]

yeah that's what I meant, ignore my ambiguity refer to shinjitsuzx's posts.


If you question the supervisors, they can sometimes go find someone that *does* know. eg in physics U3 2007 they relayed back to us after 10 minutes that what a kid asked one of the supervisors if something was a mistake actually was a mistake (2 typing/printing errors and one incorrect number in that paper )


dusty: people aren't perfect, there's only meant to be one correct MC answer. This is why the people who write the paper and the people who determine how it will be marked are independent from each other, if the assessors deem that 2 MC answers could be correct thats where it comes from.

[Rosie]

Can someone please explain these statements to me:


If the amount of energy released when products are formed is greater than the activation energy requirement, there will be an overall release of energy, and the reaction is classified as exothermic.



If the amount of energy released when products are formed is less than the activation energy requirement, overall, energy was absorbed from the surroundings, and the reaction is classified as endothermic.

[bturville]

The red line is activation energy, and the green line ΔH




This stuff should be explained quite well in your text.