O-Level Chemistry

[Zhou.012490s Studio]

Hi another question to attempt:


A liquid is to be electrolysed and two test tubes full of the liquid are placed over carbon electrodes. The diagram below shows the apparatus after a current has passed for some time.

http://i57.tinypic.com/2ikccx1.jpg\">

Which one of the following could be the liquid undergoing electrolysis?

A. concentrated aqueous copper (II) sulfate
B. concentrated aqueous sodium chloride
C. an aqueous solution of sugar
D. dilute hydrochloric acid

(p/s: look carefully on the volume displaced by gasses in both test tubes)


:? :idea:

[Zhou.012490s Studio]

Another question to attempt:


The equation for the reductino of iron ore in the blast furnace is:

Fe2O3 (s) + 3CO2 --------> 2Fe (l) + 3CO2 (g)

When 40 tonnes of the iron ore were reduced, 14 tonnes of molten iron were produced. What is the percentage of this reduction?

_____

We enter to learn, leave to achieve. Happy trying!

[Zhou.012490s Studio]

Zhou's Studio:

Hi another question to attempt:


A liquid is to be electrolysed and two test tubes full of the liquid are placed over carbon electrodes. The diagram below shows the apparatus after a current has passed for some time.

http://i57.tinypic.com/2ikccx1.jpg\">

Which one of the following could be the liquid undergoing electrolysis?

A. concentrated aqueous copper (II) sulfate
B. concentrated aqueous sodium chloride
C. an aqueous solution of sugar
D. dilute hydrochloric acid


Hi hi the above answer is D.

To differentiate the answers, look for the hints concentrated and dilute as both types of solution will yield different products at anode and cathode. If is concentrated sodium chloride, Cl2 gas will be discharged at anode and not Oxygen gas.
writing half equation:
Anode: 2Cl- ----> Cl2 +2e
Cathode: 2H+ + 2e. ------> H2

So 1 mole of chlorine gas release will also release 2 electrons and hence 1 mole of hydrogen gas. Hence this is not the answer as the test tube of anode and cathode has the gas liberated at 1:2

For option A, copper will deposited on the anode and not gas, this is because copper is preferable discharged compared to H+.

For option D, because it is dilute HCL, the half equation will be:

Anode: 4OH- ------> 2H2O + O2 + 4e
Cathode: 2H+ + 2e -------> H2

As 4e releases for 1 mole of O2, (hypothetically) 2 mole of H2 gas will be released. Both gases are not soluble, hence this explains the volume of gas collected in anode and cathode is 1:2.


For this question, students has to be aware on the difference of dilute and concentrated ions affect the yields. Also the preferential discharge nature of both cations and anions. Lastly stoichiometry. But very important is read the questions and the diagram carefully. Analyze and think through step by step. :rahrah:

Lastly And most importantly, practise and practise!






(p/s: look carefully on the volume displaced by gasses in both test tubes)


:? :idea:

[Zhou.012490s Studio]

Zhou's Studio:

Another question to attempt:


The equation for the reductino of iron ore in the blast furnace is:

Fe2O3 (s) + 3CO2 --------> 2Fe (l) + 3CO2 (g)

When 40 tonnes of the iron ore were reduced, 14 tonnes of molten iron were produced. What is the percentage yield of this reduction?

_____

We enter to learn, leave to achieve. Happy trying!

Here is the answer:

1. Read the question carefully, what is the question asking?
YES, it means what is the percentage of recovery of Iron (Fe) from iron ore.

2. What should we do?
Calculate the mass of Fe in iron ore first.

Mass percentage of Fe in iron ore = Mr of Fe / Total Mr of Fe2O3
= 56 x2 / (56x2)+(16x3)

= 0.70 x 100% = 70%
Mass of Fe in Iron ore = 0.7 x 40 tonnes

= 28 tonnes

3. Now calculate the percent yield = mass of product / mass of reactants

therefore, percentage yield will be = 14 tonnes / 28 tonnes
= 0.5 x 100% = 50%


Hope you all understand it.

[PurelyChemistry]

Need help with O level Chemistry?

Send me a text at 82620671 to join WA chatgroup for Q&A for Chemistry.

[zbear]

PSLEmom:

Any holiday lessons for Sec 4 / O level batch of 2015 at any of the tuition centres?

PSLEmom, is it time to change your name to Olevelmum????

[Misaka]

Hi, may i know if anyone has the suggested answer scheme for O Level Chemistry MCQ 2014? My answers are as follows,


1-10 : CAABAACDDB

11-20 : DDBADABCCC

21-30 : CDCDCABCAC

31-40 : BBBBDBCBDD

[KSP2013777]

Somebody can help me to know my understanding correct or not?


Question - Is the number of atoms in 0.5 mol of oxygen gas and 0.5 mol of iron the same?

I think answer is not correct because Oxygen gas is made up of molecules (O2)

That means, 0.5 mol of oxygen gas have 0.5 x 2 x 6 x 10^23 = 6 x 10^23 oxygen atoms.

But, 0.5 mol of iron have 0.5 x 6 x 10^23 = 3 x 10^23 iron atoms.

So, answer is no, am I right? pls ans when u hv time.

[soseducation]

KSP2013777:

Somebody can help me to know my understanding correct or not?


Question - Is the number of atoms in 0.5 mol of oxygen gas and 0.5 mol of iron the same?

I think answer is not correct because Oxygen gas is made up of molecules (O2)

That means, 0.5 mol of oxygen gas have 0.5 x 2 x 6 x 10^23 = 6 x 10^23 oxygen atoms.

But, 0.5 mol of iron have 0.5 x 6 x 10^23 = 3 x 10^23 iron atoms.

So, answer is no, am I right? pls ans when u hv time.

Yes, you are right. The key word in the question is ATOMS. not molecules not ions. Oxygen itself is made up of two atoms since it is diatomic and you have to take that into consideration.

[KSP2013777]

soseducation, thanks. I so scared my answer is wrong