multiple choice questions (1 Viewer)

[blackratpoo]

please help.

1. A student diluted a solution of an acid by mixing 10 mL with 90 mL of water. If the
original solution had a pH of 2.1, what will the final pH of the solution be?
(A) 1.1
(B) 2.1
(C) 3.1
(D) 4.1

2. Consider the equilibrium reaction: C6H5COOH (aq) + H2O (l)---> C6H5COO-(aq) + H3O+ (aq)
Which one of the following would decrease the C6H5COO- ion concentration in solution?
(A) adding H2
(B) adding HCl
(C) adding NaOH
(D) adding C6H5COOH

3. A piece of zinc is placed into copper(II) nitrate solution in a test tube. Which one of the following changes would occur?
(A) The concentration of nitrate ions would decrease.
(B) The colour of the solution would darken.
(C) The concentration of nitrate ions would increase.
(D) The colour of the solution would fade.

An unknown solid was analysed by a number of tests, the results of which are described below.
- The solid did not react when HNO3 (aq) was added to a sample.
- When Ba(NO3)2 (aq) was added to a solution of the solid, no observable change occurred.
- When AgNO3 (aq) was added to a solution of the solid, a cream precipitate formed. The precipitate dissolved in excess nitric acid.
- When a small sample of the solid was placed into a Bunsen flame, a flash of red colour was observed.
Which one of the following chemicals would behave in a similar way when analysed with the same set of tests?
(A) barium chloride
(B) barium phosphate
(C) calcium chloride
(D) calcium phosphate

Thankyou for your help.

 

[adomad]

please help.

1. A student diluted a solution of an acid by mixing 10 mL with 90 mL of water. If the
original solution had a pH of 2.1, what will the final pH of the solution be?
(A) 1.1
(B) 2.1
(C) 3.1
(D) 4.1

2. Consider the equilibrium reaction: C6H5COOH (aq) + H2O (l)<---> C6H5COO-(aq) + H3O+ (aq)
Which one of the following would decrease the C6H5COO- ion concentration in solution?
(A) adding H2
(B) adding HCl
(C) adding NaOH
(D) adding C6H5COOH

guessing 3 A

 

[blackratpoo]

can you explain your answers? epsecially 1. i dont get dilution.

 

[Pwnage101]

One way to think about it is, it is a dilution by a factor of 10 - we know diluting an acid makes pH increase - pH is a logarithmic scale, so a 10-fold change in concentration means a 1-point change in pH. Thus, pH should increase by 1 - to 2.1+1=3.1.

A more rigorous apporach would be as follows:

pH(original)=2.1
V(original)=10mL=0.01L

c=[H+]=10^-pH ---> [H+](Original) = (10^-2.1)

n(original)=cV=(10^-2.1)*0.01

n(final)=n(original) [nothing new has been added other than water = so H+ stays the same]

V(final)=100mL=0.1L

c(final)=[H+]=n/V=(10^-2.1)*0.01/0.1=(10^-2.1)*0.1

pH(final)= -log_10[H+] = -log_10{(10^-2.1)*0.1}=-log_10(7.943282347*10^-4)=3.1 as above.

 

[Miss Successful]

I reckon Q3 is D because if you have a look at the reactivity series, Zn is more reactive than Cu so when you add Zn solid to Copper nitrate, the Zn will displace the Cu:

Zn(s) + Cu(NO3)2 (aq) --> Zn(NO3)2 (aq) + Cu(s)

The presence of Cu ions make a solution blue so if they're removed, the solution loses its colour. Hence D.

Q2: it's Le Chatelier's principle. A doesn't do anything coz there's no H2 involved in the reaction; B is right coz when you add HCl, it's like adding H3O+ [coz HCl becomes H3O+ and Cl- in solution] so favours the reverse reaction; C&D both favour the forward reaction [C- coz you're incr. the concentration of reactants on the left and D- coz when you add OH-, it'll react with the H3O+ to form water, which is also a reactant on the left, so same mechanism as C]

not sure about q4. i'd probably guess A or C coz it's making silver precipitate..hmm

hope that helped

 

[randomnessss]

please help.

1. A student diluted a solution of an acid by mixing 10 mL with 90 mL of water. If the
original solution had a pH of 2.1, what will the final pH of the solution be?
(A) 1.1
(B) 2.1
(C) 3.1
(D) 4.1

2. Consider the equilibrium reaction: C6H5COOH (aq) + H2O (l)---> C6H5COO-(aq) + H3O+ (aq)
Which one of the following would decrease the C6H5COO- ion concentration in solution?
(A) adding H2
(B) adding HCl
(C) adding NaOH
(D) adding C6H5COOH

3. A piece of zinc is placed into copper(II) nitrate solution in a test tube. Which one of the following changes would occur?
(A) The concentration of nitrate ions would decrease.
(B) The colour of the solution would darken.
(C) The concentration of nitrate ions would increase.
(D) The colour of the solution would fade.

An unknown solid was analysed by a number of tests, the results of which are described below.
- The solid did not react when HNO3 (aq) was added to a sample.
- When Ba(NO3)2 (aq) was added to a solution of the solid, no observable change occurred.
- When AgNO3 (aq) was added to a solution of the solid, a cream precipitate formed. The precipitate dissolved in excess nitric acid.
- When a small sample of the solid was placed into a Bunsen flame, a flash of red colour was observed.
Which one of the following chemicals would behave in a similar way when analysed with the same set of tests?
(A) barium chloride
(B) barium phosphate
(C) calcium chloride
(D) calcium phosphate

Hmmm...I think these are the answers.