Chemistry MCQ questions

anpanman SG

What do both an atom and an ion of the isoptope of sodium Na, contain?

A 11 electrons

B 12 neutrons

C 23 protons

D 23 neutrons

Ans: B I just don't understand how the answer can be B. Will someone please explain?

2. When testing for a chloride ion using silver nitrate, the solution must be acidified with dilute nitric acid

What is the purpose of the nitric acid?

A to act as a catalyst

B to oxidize the chloride ion

C to prevent precipitation of silver carbonate

D to prevent the decomposition of any silver chloride formed

I have got 2 TYS and one gives the answer as B while the other gives the answer as C. I think the answer should be B, after all it's impossible to get a "carbonate" from this reaction... but somehow or rather, i do remember my teacher saying that adding the nitric acid is for prevention of the precipitation (or something like that)

qdtimes2 SG

1. refering to the periodic table, Na's relative atomic mass is 23 and its proton/atomic number is 11. Since the sum of protons and neutrons is 23, the no. of neutrons is 12. choice A is out because sodium ions lose 1 electron.

2. to prevent precipitation of the carbonate salt.

this because the solution may contain carbonate ions too. When silver nitrate is added, an insoluble ppt like silver chloride may be formed too. So this makes it hard to tell whether the ppt is silver carbonate or silver chloride, hence we also cannot whether the solution really contains chloride ions.

anpanman SG

2. to prevent precipitation of the carbonate salt.

thanks, but i still don't understand question 1. i mean the question's asking for the similarity between both atom and an ion of the isotope Na. I though isotope itself already has different neutrons(in O's syllabus)?

TrueHeart SG

Hi anpanman,

I believe the question is not well-phrased enough. It is actually asking for the similarity between an atom of the isotope of sodium and an ion of the isotope of sodium. I hope this clears your misunderstanding.

Cheers.

anpanman SG

ok this's a lame question but i shall ask anyway.

which ions are discharged when concentrated aqueous sodium chloride is electrolysed?

so is the solution concentrated or aqueous ?

Thanks TrueHeart

dkcx SG

ok this's a lame question but i shall ask anyway.

which ions are discharged when concentrated aqueous sodium chloride is electrolysed?

so is the solution concentrated or aqueous ?

Thanks TrueHeart

Aqueous just means dissolve in water. Concentrated or not, both can be aqueous

absol SG

so the only difference is still whether it is dilute or concentrated right?

dkcx SG

so the only difference is still whether it is dilute or concentrated right?

Yeah. Only molten is different from aqueous

TrueHeart SG

ok this's a lame question but i shall ask anyway.

which ions are discharged when concentrated aqueous sodium chloride is electrolysed?

so is the solution concentrated or aqueous ?

Thanks TrueHeart

Hi anpanman,

I believe Hydrogen and Chloride ions will be discharged.

Cheers.

yiha093 SG

anpanman SG

Hmm, so is it concentrated or aqueous? I mean from TrueHeart's answer, I can roughly infer that it is from a concentrated solution since only a concentrated one discharges chloride ions.

anpanman SG

i encountered a few more questions while doing the MCQ section.

1. an unrelated question. How do I know how to go about naming an ester? Can someone give me an example? I mean I know alcohol's name comes first before carboxylic acid. But if the question gives me an equation and instantly asks me to name the substance (say, a complicated ester formula) , how can i swiftly recognize it?

Lost 1 mark to such a question in my Paper 2.

2. why is it that when you add water to NaOH, it becomes a good conductor of electricity? (is it to disscoiate the ions? you mean NaOH is not already ionized just like HCl acid is?)

3. When NH3 and H2SO4 react to form ammonium sulfate, why is it considered neither an oxidation nor reduction reaction?

N does experience increase in oxidation state from +3 in NH3 to +5 in (NH4)2SO4

(PS: I take it as NH4 + and from there i calculated Nitrogen's oxidation state in ammonium sulfate. Please correct me if I am wrong)

4. why is it that magnesium nitrate cannot react with dilute H2SO4?

All the above are MCQ questions (except Question 1). The reason why I didn't type out the entire question is because I managed to do the above questions correctly using elimination method. But I want to know why certain reactions occur and others do not.

Thanks!

dkcx SG

A concentrated solution is still aqueous, there is no either or. I don't get whats your problem with it.

1) The C chain that is single bonded to the O belongs to the alcohol while the chain that is bonded to the C=O is part of the acid.

2) All acids and bases have to be in water before they can dissociate to form H+ or OH-. If they are in any other solution, they will generally not dissociate.

3) N in NH3 is -3 not +3 and a -3 is the same as a +5.

4) Mg(NO3)2 is slightly acidic salt and thus would not react with an acid.

anpanman SG

3) N in NH3 is -3 not +3 and a -3 is the same as a +5.

4) Mg(NO3)2 is slightly acidic salt and thus would not react with an acid.

wow, i didn't know that. that wasn't taught in school. thanks!

can you please explain further for the naming of esters from a formula? I still don't really get it . Perhaps you can give me a formula and let me try naming it? Thanks.

TrueHeart SG

wow, i didn't know that. that wasn't taught in school. thanks!

can you please explain further for the naming of esters from a formula? I still don't really get it . Perhaps you can give me a formula and let me try naming it? Thanks.

Hi anpanman,

Do scroll down and have a look at the Ester naming section.

http://www.chemguide.co.uk/basicorg/conventions/names2.html

You will have to figure out which acid and alkyl group forms up the particular ester first before you can name it.

Cheers.

yiha093 SG

wow, i didn't know that. that wasn't taught in school. thanks!

can you please explain further for the naming of esters from a formula? I still don't really get it . Perhaps you can give me a formula and let me try naming it? Thanks.

this isnt in syllabus, its like the components of the salt that makes it acidic or alkalinic, eg, potassium nitrate is alkaline, bla blah

for O lv

ALL salt= NEUTRAL.

dkcx SG

this isnt in syllabus, its like the components of the salt that makes it acidic or alkalinic, eg, potassium nitrate is alkaline, bla blah

for O lv

ALL salt= NEUTRAL.

KNO3 is a neutral salt not alkaline.

A rough way to determine whether a salt is acidic or neutral is to look at the 2 'components' that form it.

If its form by a strong acid such as HNO3 with a weak base such as Ca(OH)2, the salt obtained Ca(NO3)2 is slightly acidic due to NO3 being slightly more acidic than the Ca is basic. The same goes for NaOH with H2CO3 would give a more basic salt since Na is a stronger base than CO3 is an acid.

This might not be true for all salts but its generally good enough to give a rough idea if its ever required.