HSC Chemistry Question Thread

[jakesilove]

Hey guys, this is just more of a confirmation for one of the conditions implemented in esterification.

For this reaction, I remember reading that the acid and alkanol are used in a 1:3 ratio even if the mol ratio is 1:1. (e.g. ethanoic acid and ethanol) So is this excess used so that as per LCP the equilibrium will shift to the products side as well as to make the acid a limiting reagent?
Also, does it matter whether the acid/alkanol is used in excess, like is one more expensive than the other?

Cheers.

The things you've discussed are WAYYYY beyond the curriculum, and I have no idea if they're true. I really wouldn't worry about it!

[RuiAce]

The things you've discussed are WAYYYY beyond the curriculum, and I have no idea if they're true. I really wouldn't worry about it!

I think he's right though. I remember having that sort of scenario when I did the experiment.

[ProfLayton2000]

When doing calculations where are your given info on two products, how would formally show which one is the limiting reagent?

[RuiAce]

When doing calculations where are your given info on two products, how would formally show which one is the limiting reagent?

Assuming you meant info on two reactants, not products.

Figure out the number of moles of each reagent first, then write out the balanced equation to determine your mole ratio.

Using your mole ratio, try to figure out which one would get used up. Then you don't have to be too formal, you can just say since 1 mol of X reacts with 2 mol of Y, X is the excess reagent and Y is the limiting reagent (or something even more simpler than that)

[angiezhang9]

there's a multiple choice question in the cambridge checkpoints book which i don't understand.

"Carbon-12, carbon-13 and carbon -14 are isotopes of carbon. Atoms of these isotopes will differ in:
a) the number of protons in their nuclei
b) their electron configuration
c) their chemical properties
d) their physical properties

I thought the answer was c but the book says that the answer is d.

[Yasminpotts1105]

Does anyone have a good link or way of knowing how to know which numbers to put out the front or in the middle of alkanes and alkenes?

[RuiAce]

there's a multiple choice question in the cambridge checkpoints book which i don't understand.

"Carbon-12, carbon-13 and carbon -14 are isotopes of carbon. Atoms of these isotopes will differ in:
a) the number of protons in their nuclei
b) their electron configuration
c) their chemical properties
d) their physical properties

I thought the answer was c but the book says that the answer is d.

Bit dodgy and pushing the boundaries of the HSC course.

Obviously A and B are wrong, but isotopic behaviour is considered to be physics.

Carbon 14 will react the same way as carbon 12 will in any chemical reaction. Their reactivities and etc. do not change. Carbon 14 just undergoes radioactive decay.

Radioactive decay isn't considered chemical as it doesn't involve any real chemistry. It's just what goes on inside the nucleus of the atom.

[Yasminpotts1105]

I was just wondering what people think about if how well you do in chemistry depends on your teacher or not?
Because I am so passionate about science and willing to put in effort but my teacher just keeps explaining things in ways I cannot understand and anything I learn is outside of his classes.
Or any advice?

[RuiAce]

Does anyone have a good link or way of knowing how to know which numbers to put out the front or in the middle of alkanes and alkenes?

I don't know what number you're putting in front of an alkane.

For an alkene, the number depends on where the double bond is. The number corresponds to the first possible carbon atom with a double bond attached.

E.g. Pretending that the H's are all there (not typed due to inconvenience), consider

C-C=C-C-C-C-C-C
and
C-C-C-C=C-C-C-C

In the first one, the double bond starts on carbon #2.
In the second one, the double bond starts on carbon #4.

So the first one is 2-octene and the second one is 4-octene

Trap:
C-C-C-C-C-C=C-C
is also 2-octene. Because we are allowed to count from left to right, or right to left. We count, in a way, so that the number is always as small as possible.



Otherwise, you need to provide a question.

I was just wondering what people think about if how well you do in chemistry depends on your teacher or not?
Because I am so passionate about science and willing to put in effort but my teacher just keeps explaining things in ways I cannot understand and anything I learn is outside of his classes.
Or any advice?

No. Unless you have a tendency to rely on teachers they aren't important if you can put in the effort to get it yourself. That's with every subject and not just chemistry.

[angiezhang9]

Bit dodgy and pushing the boundaries of the HSC course.

Obviously A and B are wrong, but isotopic behaviour is considered to be physics.

Carbon 14 will react the same way as carbon 12 will in any chemical reaction. Their reactivities and etc. do not change. Carbon 14 just undergoes radioactive decay.

Radioactive decay isn't considered chemical as it doesn't involve any real chemistry. It's just what goes on inside the nucleus of the atom.

THANK YOU!

[medicinestudent]

not sure if this question has been asked yet?

A sample of pond water from a contaminated site was analysed to determine the
concentration of lead ions using the following procedure.
•    A measuring cylinder was used to collect a 50 mL sample from the pond.
•    The sample was placed in a clean dry beaker.
•    25.0 mL of 0.200 mol L−1 sodium chloride solution was added to the sample.
•    The precipitate of lead(II) chloride that formed was filtered, dried and weighed. It had
a mass of 0.13 g.

2015 hsc mc 19
What was the concentration of lead ions in the sample?
(A)        5.0 × 10−3 mol L−1
(B)        5.8 × 10−3 mol L−1
(C)        9.3 × 10−3 mol L−1
(D)       10.7 × 10−3 mol L−1

i can't do ppm or concentration of ions in solution!!! no idea where to start? moles? volume? c1v1?

[RuiAce]

not sure if this question has been asked yet?

A sample of pond water from a contaminated site was analysed to determine the
concentration of lead ions using the following procedure.
•    A measuring cylinder was used to collect a 50 mL sample from the pond.
•    The sample was placed in a clean dry beaker.
•    25.0 mL of 0.200 mol L−1 sodium chloride solution was added to the sample.
•    The precipitate of lead(II) chloride that formed was filtered, dried and weighed. It had
a mass of 0.13 g.

2015 hsc mc 19
What was the concentration of lead ions in the sample?
(A)        5.0 × 10−3 mol L−1
(B)        5.8 × 10−3 mol L−1
(C)        9.3 × 10−3 mol L−1
(D)       10.7 × 10−3 mol L−1

i can't do ppm or concentration of ions in solution!!! no idea where to start? moles? volume? c1v1?

Write out the relevant equation to determine the species and mole ratio

Pb²⁺ + 2 NaCl -> PbCl_2(s) + 2 Na⁺

We should work backwards since they give us information on the product

n_PbCl2 = 0.13/(207.2 + 35.45*2) = 4.674... * 10^-4 mol

Briefly check the moles of NaCl we had added:
n_{NaCl added}=0.025*0.2 = 5*10^-3 mol
Implying (despite the mole ratio) that NaCl was the excess reagent after all, and Pb²⁺ was the limiting reagent.

Due to the mole ratio, we have n_Pb²⁺ = 4.674... * 10^-4 mol

Hence, via C=n/V

C = 4.674... * 10^-4/0.05 = 9,349... * 10^-3 mol L^-1

Answer: C

[Sssssrr]

for the dot point in industrial chemistry - "identify, given a flow chart, the sequence of steps used in the solvay process to describe the chemistry involved in: brine purification, hydrogen carbonate formation, ammonia recovery", does this mean if they ask a question on it in the hsc they;; put in a flowchart, or do we have to know the flowchart?

[RuiAce]

for the dot point in industrial chemistry - "identify, given a flow chart, the sequence of steps used in the solvay process to describe the chemistry involved in: brine purification, hydrogen carbonate formation, ammonia recovery", does this mean if they ask a question on it in the hsc they;; put in a flowchart, or do we have to know the flowchart?

Every instance (not even necessarily this dot point) in the chemistry course where they asked you to draw a flow chart, you had to draw it yourself.

Yet I've never seen that dot point examined like that either. I recall writing a 7 marker on the Solvay process and its procedures but never the need to actually draw it out.

[nibblez16]

Hello, for this question can we identify (A) as starch?