HSC Chemistry Question Thread

[Bubbly_bluey]

Just to confirm, is the water and ethanol gas or liquid during hydration and dehydration process?

im pretty sure liquid for both

[Bubbly_bluey]

hey guys! can someone explain or illustrate a way to understand why a conjugate base of Bronsted Lowry acid always has a lone pair?
thanks

[cxmplete]

Hi,
Would it be okay if someone explained titration calculations? Our teacher just rushed over it and I haven't really grasped it.

[Bubbly_bluey]

Hi,
Would it be okay if someone explained titration calculations? Our teacher just rushed over it and I haven't really grasped it.

Hey! I usually go by this method when tackling titration questions. Of course it pretty general but if you have a specific question i may be able to help you out
1) I always start with a diagram and label everything im given in the question like how much volume used, concentration.
So for example in burette: what solution is it? concentration? volume? can i work out the number of moles?
2) write out a balanced equation of the reaction
3) calculate any missing information and add it to your diagram.
4) Depending on the question, this should be enough info find concentration of unknown. From balanced equation deduce the stiochimetric ratios. Use the number of moles you calculated in step 1 to find the number of moles for unknown. Now for the unknown you have both volume and number of moles so just use C=n/v to find concentration.

Again this may or may not answer ur question (sorry)

[Alalamc]

Hey Guys
I was hoping that someone could actually see if Q2 of PH titrations answers are correct because I keep getting the PH's in decimals and also can someone help with Q1. would be great help!
Q1
Calculate the pH in the titration of 10.00mL of 1.0M HCl with 1.0M NaOH after the addition of 10mL NaOH?
Balanced equation shows reaction ratio of 1:1

n(OH-)= 1.0mol/L x 0.010= 0.010mol
Excess n[H+]= 0.01-0.010= 0
Total Volume: 0.020 L
M= 0/0.020= 0

pH= -log[0]
= ERROR

Q2
Calculate the pH in the titration of 10.00mL of 1.0M HCl with 1.0M NaOH after the addition of 2.0mL NaOH?
Balanced equation shows reaction ratio of 1:1

n(OH-)= 1.0mol/L x 0.002= 0.002mol
Excess n[H+]= 0.01-0.002= 0.008
Total Volume: 0.012 L
M= 0.008/0.012= 0.67

pH= -log[0.67]
= 0.18

[kiwiberry]

Hey Guys
I was hoping that someone could actually see if Q2 of PH titrations answers are correct because I keep getting the PH's in decimals and also can someone help with Q1. would be great help!
Q1
Calculate the pH in the titration of 10.00mL of 1.0M HCl with 1.0M NaOH after the addition of 10mL NaOH?
Balanced equation shows reaction ratio of 1:1

n(OH-)= 1.0mol/L x 0.010= 0.010mol
Excess n[H+]= 0.01-0.010= 0
Total Volume: 0.020 L
M= 0/0.020= 0

pH= -log[0]
= ERROR

Q2
Calculate the pH in the titration of 10.00mL of 1.0M HCl with 1.0M NaOH after the addition of 2.0mL NaOH?
Balanced equation shows reaction ratio of 1:1

n(OH-)= 1.0mol/L x 0.002= 0.002mol
Excess n[H+]= 0.01-0.002= 0.008
Total Volume: 0.012 L
M= 0.008/0.012= 0.67

pH= -log[0.67]
= 0.18

1) If there is no excess H+, the solution will be neutral! Hence it will have a pH of 7
2) You've got it right! The pH scale isn't restricted to whole numbers

[Kekemato_BAP]

Just to confirm, is the water and ethanol gas or liquid during hydration and dehydration process?

The water in hydration is steam and ethene is naturally a gas. Both hydration and dehydration are conducted at temperatures of 170-300 degrees Celsius so I assume they're all going to be gas since it exceeds their boiling points.

[Bubbly_bluey]

The water in hydration is steam and ethene is naturally a gas. Both hydration and dehydration are conducted at temperatures of 170-300 degrees Celsius so I assume they're all going to be gas since it exceeds their boiling points.

yes sorry you're right! a terrible mistake on my part. I did a quick search and found that:
In dehydration
ethanol (g) --> ethylene (g) + H2O (g) because it is occuring at high temperatures as you said.
In Hydration: the process in occuring at 300 degrees
ethylene(g) + H2O (l)--> ethanol (g)
water in this case could also be a gas; it can start off as a liquid and turn into steam.

[Bubbly_bluey]

Hey guys! So I did a fermentation prac a while ago, we added sugar, yeast and orange juice into our test tubes and left it to ferment for a few days. Does anyone know a possible reason why orange juice was added?seems odd lol.
 Thanks

[jakesilove]

Hey guys! So I did a fermentation prac a while ago, we added sugar, yeast and orange juice into our test tubes and left it to ferment for a few days. Does anyone know a possible reason why orange juice was added?seems odd lol.
 Thanks

Aha that is pretty bloody odd. Basically, what you need is some sort of substance to ferment (eg. sugar), and a fermentor (ie. Yeast). Potentially the Orange Juice was just like extra 'food' to be digested and fermented by the Yeast? Definitely an odd experiment, I would recommend just learning the standard experiment (which involves putting glucose and yeast into a beaker, along with standard conditions re heat, anaerobic etc).

Seriously weird prac aha

[Alalamc]

So sorry to be here again! its just that I don't have access to teachers and I don't even go to school anymore, if someone can explain this to me it'll be real good! I worked out Q1 (hope its right)
1) 20.0 g of N2 and 20.0 g of O2 are placed in a metal container with a volume of 12.0 L at a temperature of 200°C. What is the pressure in the container?
Pressure of N2:
n: 20g/ 28.0= 0.71
V: 12L
T: 200 degrees or 473K
P= 0.71x8.31x473/12
P= 232.6kPa

Pressure of O2:
n= 20g/32.0= 0.625
P= 0.63x8.31x473/12
P= 206.4kPa

Total Pressure: 206.4+ 232.6= 439kPa
 2)
A spark causes the gases in question 14 to react to form NO2. Assuming the reaction goes to completion and that there is no charge in temperature or volume, what is the pressure in the container after reaction?

[jakesilove]

So sorry to be here again! its just that I don't have access to teachers and I don't even go to school anymore, if someone can explain this to me it'll be real good! I worked out Q1 (hope its right)
1) 20.0 g of N2 and 20.0 g of O2 are placed in a metal container with a volume of 12.0 L at a temperature of 200°C. What is the pressure in the container?
Pressure of N2:
n: 20g/ 28.0= 0.71
V: 12L
T: 200 degrees or 473K
P= 0.71x8.31x473/12
P= 232.6kPa

Pressure of O2:
n= 20g/32.0= 0.625
P= 0.63x8.31x473/12
P= 206.4kPa

Total Pressure: 206.4+ 232.6= 439kPa
 2)
A spark causes the gases in question 14 to react to form NO2. Assuming the reaction goes to completion and that there is no charge in temperature or volume, what is the pressure in the container after reaction?

Hey! This out pretty far outside the HSC curriculum, but I've done PV=nRT in uni and it looks like the first question was done correctly! For the second one, we start by writing out a chemical equation.



Clearly, the molar ratio is 1:2. Now, let's work out the moles of each of the substance. Actually, you've already done that!




For everyone one mole of Nitrogen, two moles of Oxygen is required. So, Oxygen will be the limiting reagent (ie. it will be completely reacted before the Nitrogen is completely reacted). So, if 0.625 moles of Oxygen is reacted, nitrogen will react in HALF of the molar quantity (0.3125 moles).

So, we know that there are 0 moles of Oxygen, 0.71-0.3125=0.3975 moles of Nitrogen, and 0.625 moles of NO2 in the final system. Use the partial pressure technique above to find the total pressure!

[Bubbly_bluey]

Hey guys!
Two beakers X and Y; either contains 0.1mol/L of HCL or 0.1mol/ acetic acid. A student performs two tests:
test 1: stuck a probe metre in to determine the pH
test2: add a volume of o.1mol/L of NaOH needed to neutralise each acid and using phenolpthalein indicator.
Assess the effectiveness of determining which acid is in which beaker when performing the two tests.
I was able to do test 1 but I need help for test 2. If HCL + NaOH--> NaCl + H2O then wouldn't the solution turn colourless and if CH3COOH + NaOH --> CH3COONa + H2O be purple (slightly basic salt?)
Thanks

[J.B]

Hi I was just wondering in Q6 of 2005 HSC exam, how do you know that the answer is A and not D? And do we have to know certain esters that are used for flavouring or solvents?

[Bubbly_bluey]

Hi I was just wondering in Q6 of 2005 HSC exam, how do you know that the answer is A and not D? And do we have to know certain esters that are used for flavouring or solvents?

(Image removed from quote.)

Hey! the answer is not D because you would have done esters I believe? So ethly propanoate is an ester that is produced from an alkanol and alkanoic acids.Usually esters produce nice fruity smells that are commonly used for perfumes, cosemetics and flavourings. They are not usually used for dissolving anything (I'm not sure) just mainly to produce smells.
As for the second question, it wouldnt hurt to know one or two just in case. Use as an example for long responses? Generally they wouldn't ask for specific flavouring come from which reactants.
Hope this helps