HSC Chemistry Question Thread

[itssona]

WHeey, which of the following best explains the use of solder to join metal pipes and wires?
is it that is has low melting point so that means you can easily melt it and join pipes together?

or that its unreactive or smth

I can't find an answer anywhere :/

[jakesilove]

WHeey, which of the following best explains the use of solder to join metal pipes and wires?
is it that is has low melting point so that means you can easily melt it and join pipes together?

or that its unreactive or smth

I can't find an answer anywhere :/

Could you provide us with the possible answers? I would actually say that it has a HIGH melting point; whilst it makes it more difficult to join the pipes together, once you've done that they will be very well attached, and you don't have to worry about them disassembling if temperatures are raised

[itssona]

Could you provide us with the possible answers? I would actually say that it has a HIGH melting point; whilst it makes it more difficult to join the pipes together, once you've done that they will be very well attached, and you don't have to worry about them disassembling if temperatures are raised

the possible answers are:
high melting point
low melting point
reactive
not reactive

But your answer actually makes so much sense- thank you Jake!!

[itssona]

another question sorry:
if i have a question: write the equation for the reaction of calcium and water (in a test)

so I would know its Ca + H2O --> OH

but how am I meant to know that hydrogen is also a product? is that like a rule?

[jakesilove]

another question sorry:
if i have a question: write the equation for the reaction of calcium and water (in a test)

so I would know its Ca + H2O --> OH

but how am I meant to know that hydrogen is also a product? is that like a rule?

Hey! There is a rule that a metal + water will result in a metal oxide + hydrogen gas. So, in this case

[itssona]

Hey! There is a rule that a metal + water will result in a metal oxide + hydrogen gas. So, in this case

omg thank you for explaining Jake:)

[anotherworld2b]

thank you for your help
I was also wondering for this reaction:

Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)

i'm not sure how Q2 is different from Q1


Q1. Using the equation for the reaction, calculate the number of moles of sodium carbonate used in each titration


Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)

n(HCl) = 0.05

n( Na2CO3 ) = ½ n(HCl)

n(Na2CO3) = ½ x 0.05 = 0.025 moles

Q2. From the equation determine the number of moles of HCL that react with each mole of sodium carbonate. Use this to determine the number of moles of HCL used in the titration.

Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)

I was also a bit confused about these questions. I am not sure what it is asking

Q5. Is the equivalence point of a reaction acidic, basic or neutral? Explain.
It should be a bit acidic?


Q7. Phenolphthalein colour change is in the vicinity of pH 9. Suppose phenolphthalein had been used in this titration:

a.Would the volume of acid required for the titration be more or less than that obtained?


less?

b.Would the calculated concentration of the acid be lower of higher than the result obtained?

All looks great! Other errors include human error (not quite reading to the calibration point when filling up the 500mL of distilled water), purity error (maybe the 2.65g was not purely the substance you were looking for), splashing of the water (potentially losing some of the substance) etc.

[jakesilove]

thank you for your help
I was also wondering for this reaction:

Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)

i'm not sure how Q2 is different from Q1


Q1. Using the equation for the reaction, calculate the number of moles of sodium carbonate used in each titration


Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)

n(HCl) = 0.05

n( Na2CO3 ) = ½ n(HCl)

n(Na2CO3) = ½ x 0.05 = 0.025 moles

Q2. From the equation determine the number of moles of HCL that react with each mole of sodium carbonate. Use this to determine the number of moles of HCL used in the titration.

Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)

I was also a bit confused about these questions. I am not sure what it is asking

Q5. Is the equivalence point of a reaction acidic, basic or neutral? Explain.
It should be a bit acidic?


Q7. Phenolphthalein colour change is in the vicinity of pH 9. Suppose phenolphthalein had been used in this titration:

a.Would the volume of acid required for the titration be more or less than that obtained?


less?

b.Would the calculated concentration of the acid be lower of higher than the result obtained?

Hey, sorry without looking at the full original question, Question 2 just seems like nonsense (since it looks like they gave you that n(HCl)=0.05 moles?)

The equivalence point depends on the titration itself. If we are titrating a strong acid and a weak base (as here), the equivalence point will be in the acidic zone (ie. pH around 4-5). 

We were titrating a strong acid and a weak base, so the equivalence point should be a lower pH. Instead, using Phenolphathein will require the pH to be raised a lot more than necessary. So, LESS acid would be used (as you've noted).

If we use LESS acid to neutralise the same amount of base, but THINK that there are more moles in that small quantity of acid, then we will calculate that the acid is stronger (less volume, but same number of moles, equals higher concentration).

[itssona]

Hey so I have a chem test coming up and we need to know all polyatomic ions and their charges - thing is I remember them but not the charges,

and in say, OH- it's simple since O is -2 and H is +1 so then adding it gives -1 which is the charge

but what about the other ones?

sorry for asking whats probably such a dumb question

[RuiAce]

Hey so I have a chem test coming up and we need to know all polyatomic ions and their charges - thing is I remember them but not the charges,

and in say, OH- it's simple since O is -2 and H is +1 so then adding it gives -1 which is the charge

but what about the other ones?

sorry for asking whats probably such a dumb question

Being blunt, you're just going to have to know them. E.g. sulfate is -3

[itssona]

Being blunt, you're just going to have to know them. E.g. sulfate is -3

great -_-

lol thanks for telling me tho, guess there's no easy way out argh XD

[RuiAce]

great -_-

lol thanks for telling me tho, guess there's no easy way out argh XD

Well, as dorky as it sounds, sporcle was what I used to memorise them.

[itssona]

Well, as dorky as it sounds, sporcle was what I used to memorise them.

OMG I haven't used sporcle in a long time- perhaps I should use it again!

Dorky but works I suppose

[itssona]

hey another dumb question but when naming covalent compounds, if its like N2O, then do we not write dinitrogen? since its diatomic can I just say nitrogen oxide?

and would i get marked down by doing so?

[RuiAce]

hey another dumb question but when naming covalent compounds, if its like N2O, then do we not write dinitrogen? since its diatomic can I just say nitrogen oxide?

and would i get marked down by doing so?

That causes confusion with NO.

NO is nitrogen monoxide
N2O is dinitrogen monoxide.