HSC Chemistry Question Thread

[kiwiberry]

Hi again (sorry)

I also have this question

The heat of combustion of propan-1-ol is 2021kJmol–1. Combustion takes place according to the equation:
2C3H7OH(l) + 9O2(g) → 6CO2(g) + 8H2O(l)
What mass of water is formed when 1530 kJ of energy is released?

thanks again

Here we can find the number of moles of propanol that were combusted since we are given heat of combustion per mol
n(propanol) = 1530/2021 = 0.75707... mol
Now you can find n(H2O) using the mole ratio in the equation, and find the mass of water using the moles!

[kiwiberry]

Again, I am sooooo sorry

can you please check my response to this question its out of 4 marks

An indicator is placed in water. The resulting solution contains the green ion, Ind −, and 4
the red molecule, HInd.
Explain why this solution can be used as an indicator. In your response, include a
suitable chemical equation that uses Ind − and Hind.
HSC 2013

Hind<--> H^+  +  Ind^-
red                        green

This solution is an indicator because it has a differentiating power (colour) between pHs. It will recognise the lower pH substance as red, and will change to green when a substance of higher pH is added. Thus, it is a valid indicator.

I wasn't really sure how to answer this q
thank you

You've got the right idea! Since the question states that the indicator has to be placed in water, it might be better to use  HInd + H2O <--> Ind- + H3O+ as your equation. Because it's 4 marks, I would expand a bit and talk about Le Chatelier's Principle, so explain how if an acid is added, [H3O+] will increase and thus equilibrium will shift left, resulting in a red colour. Vice versa for bases.

[bananna]

You've got the right idea! Since the question states that the indicator has to be placed in water, it might be better to use  HInd + H2O <--> Ind- + H3O+ as your equation. Because it's 4 marks, I would expand a bit and talk about Le Chatelier's Principle, so explain how if an acid is added, [H3O+] will increase and thus equilibrium will shift left, resulting in a red colour. Vice versa for bases.

i get it thank you

[roygbivmagic]

Hi, could you please explain why KHSO4 is an acidic salt?
I understand that in solution KHSO4 --> K+ and HSO4-
K+ cannot accept or donate protons, and HSO4- can donate its H+. But can't HSO4- also accept a H+?

[RuiAce]

Hi, could you please explain why KHSO4 is an acidic salt?
I understand that in solution KHSO4 --> K+ and HSO4-
K+ cannot accept or donate protons, and HSO4- can donate its H+. But can't HSO4- also accept a H+?

It can, but it tends not to. Whilst HSO₄^- is technically amphiprotic, in water it has a tendency to be acidic.

i.e. It has a tendency to go from HSO₄^- to SO₄^2- in water instead. Not backwards into becoming H₂SO₄ again.

[Kle123]

Could someone explain to me how this is an example of a Bronsted and Lowry acid-base reaction.

[kiwiberry]

Could someone explain to me how this is an example of a Bronsted and Lowry acid-base reaction.

The Bronsted-Lowry definition states that acids are proton donors, and bases are proton acceptors.
Here, HCl is the acid, and donates a proton to form Cl^-. NH₃, the base, accepts this proton to form NH₄⁺. These two ions combine together to form NH₄Cl

[bsdfjnlkasn]

Hi, could you please explain why KHSO4 is an acidic salt?
I understand that in solution KHSO4 --> K+ and HSO4-
K+ cannot accept or donate protons, and HSO4- can donate its H+. But can't HSO4- also accept a H+?

Hey I just wanted to extend this question to bicarbonate HCO₃

Is it also technically amphiprotic as it is can either become carbonate or carbonic acid - are we expected to know which will be favoured more generally? Also on the quoted question, what is it about K+, Na+, these non-metal ions that prevents them from accepting hydrogens? I understand Na is inert but if someone could please offer a more in-depth explanation for why not just it being so I would really appreciate it (but it's totally ok if I have to accept it for the HSC)

Thank you! 

[bsdfjnlkasn]

Hey there,

I was just wondering if there was anything particularly notable about neutral oxides which make them unable to react with acidic or basic oxides? Or are we just expected to learn some in which case are there any I'm missing from the following list? CO,NO, N₂O


More questions:

I was also wondering if I could get some clarification with how increasing the volume of a substance decreases pressure if the equilibrium can only be in a closed system?
Because if we increase volume of say water in a bottle, aren't we increasing the pressure of the air inside as the same volume is now forced to fit into a smaller area?

But for now taking that fact for granted, does an increase in pressure cause the system to favour side with fewer gas moles because it's trying to maintain the same mole ratio?

For the following where all reactants/products are gases, would increasing the volume be the same as increasing the pressure of that substance? What does pressure specifically refer to? That of the gas or the entire system?

CH₃OH (g) <--> CO (g) + 2H₂ (g)


2. Is it true that if the pressure of a system is increased the concentration of all the substances increases? Because I don't understand how pressure (only concerned with gases) influences concentration (only concerned with aqueous solutions)



Sorry I know - a lot of questions, any guidance/answers would be greatly appreciated 

[Shadowxo]

Hey there,

I was just wondering if there was anything particularly notable about neutral oxides which make them unable to react with acidic or basic oxides? Or are we just expected to learn some in which case are there any I'm missing from the following list? CO,NO, N₂O


More questions:

I was also wondering if I could get some clarification with how increasing the volume of a substance decreases pressure if the equilibrium can only be in a closed system?
Because if we increase volume of say water in a bottle, aren't we increasing the pressure of the air inside as the same volume is now forced to fit into a smaller area?

But for now taking that fact for granted, does an increase in pressure cause the system to favour side with fewer gas moles because it's trying to maintain the same mole ratio?

For the following where all reactants/products are gases, would increasing the volume be the same as increasing the pressure of that substance? What does pressure specifically refer to? That of the gas or the entire system?

CH₃OH (g) <--> CO (g) + 2H₂ (g)


2. Is it true that if the pressure of a system is increased the concentration of all the substances increases? Because I don't understand how pressure (only concerned with gases) influences concentration (only concerned with aqueous solutions)



Sorry I know - a lot of questions, any guidance/answers would be greatly appreciated 

Hi

1. Volume and pressure:
First of all, the pressure of a substance is proportional to the concentration of a substance, and so the two are often interchangeable when referring to a specific substance.
With gases, increasing volume is increasing the volume of the container holding the gas. This means the same number of moles of gas occupy more space, therefore as c=n/V, and V increases while n stays the same, the concentration decreases, and therefore the pressure decreases (molecules don't hit the sides of the container as much - more space in the container).
Yes, an increase in pressure causes the system to favour the side with fewer gas molecules. This is due to wanting to maintain equilibrium - using the K value (not sure if you know this) you can see it'll favour the side with less gas molecules. This can also be explained by Le Chatlier's principle, it wants to partially oppose the change by decreasing the overall pressure of the system.

Pressure is proportional to concentration for gases, so a decrease in pressure is the same as saying a decrease in concentration. Pressure can refer to either the overall system or the individual substances, they'll usually specify. Often it'll refer to both. If it says "pressure of [substance]" it'll refer to that substance, if it says "the pressure was decreased" it'll usually refer to the whole system.
Increasing the volume would be the same as decreasing the pressure/concentration of each individual substance, and decreasing the overall pressure/concentration.

2. Concentration can be used both for aqueous solutions and gases. c=n/v, both liquids and gases have a number of moles per volume. eg a gas having 2 moles of it in a 2L container, c=n/V = 1molL^-1.  Pressure is only used for gases though.
The pressure of a system can be increased in two ways:
- Adding an inert gas. This increases the pressure of the system but doesn't affect the concentration/pressure of each individual substance, so has no effect on equilibrium.
- Decreasing volume / adding more of each substance (usually the former). This increases the pressure/concentration of both the overall system and each substance. So if the volume is decreased, the concentration of each substance increases.

Hope this helps, if anything need further clarification just ask

[bsdfjnlkasn]

Hey there,

Could I get some help with the following questions? I feel like there's a trick somewhere because my usual calculations aren't getting the correct answers - any help would be greatly appreciated 

1. To determine the concentration of ammonia in ‘cloudy ammonia’ solution, a chemist first accurately diluted 25mL (by pipette) to 500mL (volumetric flask), then titrated 25mL (by pipette) of the diluted solution with 0.151 mol/L nitric acid solution; it required 27.2mL. Calculate the molarity of the original ammonia solution.


2. 25mL of H2SO4 (aq) from a car battery was accurately diluted to 500mL. 25mL of the diluted solution was titrated with 0.206 mol/L NaOH. It required 38.8mL. Calculate the molarity of the original battery acid and its concentration in grams per litre. If 50mL of this acid were split on the bench, how much anhydrous sodium carbonate would be needed to neutralise it?

[kiwiberry]

Hey there,

Could I get some help with the following questions? I feel like there's a trick somewhere because my usual calculations aren't getting the correct answers - any help would be greatly appreciated 

1. To determine the concentration of ammonia in ‘cloudy ammonia’ solution, a chemist first accurately diluted 25mL (by pipette) to 500mL (volumetric flask), then titrated 25mL (by pipette) of the diluted solution with 0.151 mol/L nitric acid solution; it required 27.2mL. Calculate the molarity of the original ammonia solution.

2. 25mL of H2SO4 (aq) from a car battery was accurately diluted to 500mL. 25mL of the diluted solution was titrated with 0.206 mol/L NaOH. It required 38.8mL. Calculate the molarity of the original battery acid and its concentration in grams per litre. If 50mL of this acid were split on the bench, how much anhydrous sodium carbonate would be needed to neutralise it?

1. NH₃ + HNO₃ --> NH₄NO₃
We know that v(diluted NH3)=0.025 L, c(HNO3)=0.151 and v(HNO3)=0.0272 L, so we can find n(HNO3)
n(HNO3) = 0.151 x 0.0272 = 0.0041072 mol = n(NH3)
so c(diluted NH3) = 0.0041072/0.025 = 0.164288 mol/L

Now, because it was a twenty-fold dilution (500/25=20),
concentration of the original ammonia solution = 20 x 0.164288 = 3.28576 = 3.3 mol/L (2sf)

2. This is basically the same method as the first question, except you have to convert concentration in mol/L to g/L by mulitplying by molar mass. For the second part of the question, you use the concentration of H2SO4 you calculated to calculate the moles of Na2CO3

[kiwiberry]

Hey I just wanted to extend this question to bicarbonate HCO₃

Is it also technically amphiprotic as it is can either become carbonate or carbonic acid - are we expected to know which will be favoured more generally? Also on the quoted question, what is it about K+, Na+, these non-metal ions that prevents them from accepting hydrogens? I understand Na is inert but if someone could please offer a more in-depth explanation for why not just it being so I would really appreciate it (but it's totally ok if I have to accept it for the HSC)

Thank you! 

I would like some clarification on this as well! Someone told me that Na+ is extremely weak because it's the conjugate acid of NaOH, a strong base, is this right?

[bsdfjnlkasn]

I would like some clarification on this as well! Someone told me that Na+ is extremely weak because it's the conjugate acid of NaOH, a strong base, is this right?

Yeah, I see how that explanation makes sense when the equation is relevant to NaOH but not every example will have Na+'s being the result of it previously reacting with the really strong base if that makes sense? Also thank you for the calculation replies, will reattempt now 

[jakesilove]

I would like some clarification on this as well! Someone told me that Na+ is extremely weak because it's the conjugate acid of NaOH, a strong base, is this right?

To be honest, I think that at this point you just need to accept it for the purpose of the HSC and move on. I could make up any number of explanations (like the one you stated above) but I don't know which would be actually correct. Suffice to say that this is a tiny hole in the curriculum, and you won't be asked to explain something like this. Great questions though!