Author Topic: HSC Chemistry Question Thread (Read 1292846 times) Tweet Share

f_tan · « **Reply #3105 on:** October 30, 2017, 11:59:03 pm »

Can anyone please explain this question? Thank you!

winstondarmawan · « **Reply #3106 on:** October 31, 2017, 12:01:27 am »

Quote from: winstondarmawan on October 30, 2017, 05:42:38 pm

Hello!
When asked to draw glucose, is it okay to use the condensed structural formula?
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/23113018_1364885916970261_476464333_o.jpg?oh=f3edc646ed78d2cdbaab29b8e7893f92&oe=59F8A60B
Also, how recent does the recently discovered elements have to be?
TIA.

Bump.

Baylsskool · « **Reply #3107 on:** October 31, 2017, 12:19:16 am »

Hey for chapter 11 in text book, I can't find a chemist or chemical engineer to talk about for that syllabus dot point, can anyone help?

MisterNeo · « **Reply #3108 on:** October 31, 2017, 01:58:09 am »

Quote from: Mymy409 on October 30, 2017, 11:29:19 pm

How do you do questions 14 and 15 from 2009 HSC? TIA!

https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-exam-chemistry.pdf

$\ce{C6H8O7}+\ce{3NaOH}\rightarrow\ce{Na3C6H5O7}+\ce{3H2O}$ $\text{Multiply NaOH volume (0.0295L) by conc. (0.55M).}\\ \text{Moles of NaOH = 0.016225mol}\\ \text{Divide by 3 to get moles of citric acid (0.00541mol).}\\ \text{Divide moles by acid volume (0.025L) for conc.}\\ \text{which is 0.2163mol/L, then multiply by 192.12}\\ \text{The answer is 41.6g/L (B).}$
$\text{From the graph, 25C can dissolve 8mg/L.}\\ \text{In 10L, there are 80mg.}$ $\text{Convert to grams (0.08g.)}\\ \text{Divide by 32g/mol to get oxygen moles (0.0025).}\\ \text{Multiply by 24.79L/mol to get volume.}$ $\text{Answer is 0.061975L, which is A (62mL).}$

Quote from: f_tan on October 30, 2017, 11:59:03 pm

Can anyone please explain this question? Thank you!
(Image removed from quote.)

For AAS to work, you need a specific wavelength of light. It's the same wavelength from the lamp to the detector, where the only difference is the intensity of the light. Hence, the answer is A because it is the only one that has the same wavelength (615.2nm) for both intensity readings.

Quote from: winstondarmawan on October 31, 2017, 12:01:27 am

Bump.

In questions that require a drawing, it would always be the structural formula ie. diagram showing shape of glucose. The other equation you’d use is in short response as C₆H₁₂O₆. For recent discoveries, you can use the most recent you can find such as Flerovium (1998), which can be from 1950-2017 since that seems pretty recent.

toasted · « **Reply #3109 on:** October 31, 2017, 08:36:15 am »

Hey!

why are the metal acetates known as CH3COOA rather than ACH3COO where A is a metal. eg. CH3COOK

Shouldn't the cation come before the anion?

MisterNeo · « **Reply #3110 on:** October 31, 2017, 09:07:40 am »

Quote from: toasted on October 31, 2017, 08:36:15 am

Hey!

why are the metal acetates known as CH3COOA rather than ACH3COO where A is a metal. eg. CH3COOK

Shouldn't the cation come before the anion?

The hydrogen that is ionised comes from the -COOH group, which is at the right-hand side of the molecule. Since we write it as CH₃COOH, the ionised form is CH₃COO^-. The positive cation attracts to the negative ion, and forms CH₃COONa because that's where the negative charge is from where hydrogen has been ionised. You can get away with writing NaCH₃COO, but CH₃COONa is more correct since it actually shows ionic attraction.

ACCC · « **Reply #3111 on:** October 31, 2017, 09:29:22 am »

Hi! Can someone pls show me how to work out this question? Thank you

40 mL of 0.10 mol L–1 NaOH is mixed with 60 mL of 0.10 mol L–1 HCl.
What is the pH of the resulting solution?
(A) 7.0 (B) 1.7 (C) 1.4 (D) 1.2

RuiAce · « **Reply #3112 on:** October 31, 2017, 09:44:25 am »

Quote from: ACCC on October 31, 2017, 09:29:22 am

Hi! Can someone pls show me how to work out this question? Thank you

40 mL of 0.10 mol L–1 NaOH is mixed with 60 mL of 0.10 mol L–1 HCl.
What is the pH of the resulting solution?
(A) 7.0 (B) 1.7 (C) 1.4 (D) 1.2

$\text{The formula }\boxed{n=CV}\text{ tells us the}\\ \text{number of moles of each reagent.}\\ \text{Remember to work in litres.}\\ \begin{align*}n_{HCl} = 0.06\times0.10&=0.006\text{ mol}\\ n_{NaOH}=0.04\times0.10 &= 0.004\text{ mol}\end{align*}$
$\text{The relevant chemical equation is}\\ \boxed{HCl_{(aq)} + NaOH_{(aq)} \longrightarrow NaCl_{(aq)} + H_2O_{\ell}}\\ \text{so 1 mol of }HCl\text{ reacts with 1 mol of }NaOH.$
i.e. the mole ratio is 1-1.
$\text{So all }0.004\text{ mol of }NaOH\text{ will react with }0.004\text{ mol of }HCl\\ \text{giving us an excess of }0.002\text{ mol of }HCl.$
_____________________________
$\text{After mixing, our volume is now 100 mL.}\\ \text{So }n=CV\text{ states that }C_{HCl} = \frac{0.002}{0.1} = 0.02\text{ mol L}^{-1}\\ \text{which means the }pH\text{ is }-\log_{10} 0.02 = 1.698\dots$

winstondarmawan · « **Reply #3113 on:** October 31, 2017, 10:46:39 am »

Quote from: MisterNeo on October 31, 2017, 01:58:09 am

In questions that require a drawing, it would always be the structural formula ie. diagram showing shape of glucose. The other equation you’d use is in short response as C₆H₁₂O₆. For recent discoveries, you can use the most recent you can find such as Flerovium (1998), which can be from 1950-2017 since that seems pretty recent.

Thank you!
I was just wondering specifically about condensed structural formula (like the link I posted).

angelahchan · « **Reply #3114 on:** October 31, 2017, 10:55:19 am »

Hi, can someone please explain how a buffer is made? I know it consists of a weak acid and its conjugate base but for example in the 2015 paper q24b, I don't understand how the buffer is actually formed. Here's the question:
"A solution is prepared by using equal volumes and concentrations of acetic acid
and sodium acetate.
Explain how the pH of this solution would be affected by the addition of a small
amount of sodium hydroxide solution. Include an equation in your answer."

MisterNeo · « **Reply #3115 on:** October 31, 2017, 11:30:24 am »

Quote from: angelahchan on October 31, 2017, 10:55:19 am

Hi, can someone please explain how a buffer is made? I know it consists of a weak acid and its conjugate base but for example in the 2015 paper q24b, I don't understand how the buffer is actually formed. Here's the question:
"A solution is prepared by using equal volumes and concentrations of acetic acid
and sodium acetate.
Explain how the pH of this solution would be affected by the addition of a small
amount of sodium hydroxide solution. Include an equation in your answer."

$\text{When you dissolve CH3COONa, it dissociates into Na+ and CH3COO-.}\\ \text{Acetic acid partially ionises into H+ and CH3COO- in an equilibrium.}$ $\ce{CH3COOH}+\ce{H2O}⇌\ce{H3O+}+\ce{CH3COO-}\\ \text{Usually there is very little CH3COO- because of weak ionisation.}\\ \text{Hence, sodium acetate is added to get equal moles to establish a buffer.}$ $\text{When you add NaOH, it decreases the H3O+ conc.}\\ \ce{NaOH}+\ce{H3O+}\rightarrow\ce{Na+}+\ce{2H2O}\\ \text{By LCP, the system will shift to the right.}\\ \text{It will create more H3O+, minimising pH change.}$ $\text{Alternatively, if HCl was added, it will increase H3O+ conc.}\\ \ce{HCl}+\ce{H2O}\rightarrow\ce{H3O+}+\ce{Cl-}\\ \text{By LCP, the system will shift to the left.}\\ \text{It will use up some H3O+, minimising pH change.}\\ \text{This is because changes to H3O+ conc. are minimised.}$

mary123987 · « **Reply #3116 on:** October 31, 2017, 11:32:17 am »

HEY does anyone know why you cant detect anions using AAS

??

MisterNeo · « **Reply #3117 on:** October 31, 2017, 12:27:54 pm »

Quote from: mary123987 on October 31, 2017, 11:32:17 am

HEY does anyone know why you cant detect anions using AAS ??

Technically, you can for some anions. Remember that AAS functions by using a single cathode lamp that emits one specific wavelength of light unique to a specific element. The problem is that some anions are polyatomic, such as CO₃^2-. This means that multiple lamps are required to detect each element, but AAS doesn't function if more than one wavelength is present, hence the inability to detect anions. However, some anions can be detected by AAS, such as Cl^-, because they are single elements.
In the HSC, AAS is only used for metal ions since that was its original purpose to detect things like heavy metals (lead in water) and trace elements (copper in soil).

bananna · « **Reply #3118 on:** October 31, 2017, 12:51:40 pm »

Hi was just wondering for chem monitoring : how many quantitative /qualitative tests do we need to know practical-wise

is 1 each enough?

so like turbidity for quality and acidity for quantity?

thanks

also what are some other qualitative tests?

kauac · « **Reply #3119 on:** October 31, 2017, 12:52:41 pm »

Hi...

When drawing structural formulae for molecules in exam, do we need to draw individual bonds between atoms in a functional group? E.g. a bond between a oxygen and hydrogen atom in a hydroxyl group. Or can we just draw the groups in condensed form, such as OH, as long as they are in the appropriate order for bonding in the molecule?

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