VCE Chemistry Question Thread

[sweetiepi]

Could someone explain the possible safety concerns regarding the following substances used in an experiment?
H2SO4
NaOH
Bromide
Iodine
Alkanes in general

Hey there!
This MSDS database may help you!

[peachxmh]

Hiya, 1/2 question here:

A commercial concrete cleaner contains hydrochloric acid. A 25.00mL volume of cleaner was diluted to 250.0mL in a volumetric flask. A 25.00mL aliquot of 0.5000M sodium carbonate solution was placed in a conical flask. Methyl orange indicator was added and the solution was titrated with the diluted cleaner. The indicator changed permanently from yellow to pink when 23.92mL of the diluted cleaner has been added.

The equation for the reaction is:
Na₂CO₃(aq) + 2HCl(aq) -> 2NaCl(aq) + CO₂(g) + 2H₂O(l)

Calculate the concentration of hydrochloric acid in the concrete cleaner.


So far, I've figured out the mol of Na₂CO₃ (0.0125 mol), and the mol of HCl (0.025 mol) but I'm not sure where to go from here. Would greatly appreciate any help and please tell me if the mol values I've figured out are wrong haha. The answer is 9.032M.

[Sine]

Hiya, 1/2 question here:

A commercial concrete cleaner contains hydrochloric acid. A 25.00mL volume of cleaner was diluted to 250.0mL in a volumetric flask. A 25.00mL aliquot of 0.5000M sodium carbonate solution was placed in a conical flask. Methyl orange indicator was added and the solution was titrated with the diluted cleaner. The indicator changed permanently from yellow to pink when 23.92mL of the diluted cleaner has been added.

The equation for the reaction is:
Na₂CO₃(aq) + 2HCl(aq) -> 2NaCl(aq) + CO₂(g) + 2H₂O(l)

Calculate the concentration of hydrochloric acid in the concrete cleaner.


So far, I've figured out the mol of Na₂CO₃ (0.0125 mol), and the mol of HCl (0.025 mol) but I'm not sure where to go from here. Would greatly appreciate any help and please tell me if the mol values I've figured out are wrong haha. The answer is 9.032M.

Your mol values look good

So we first have the sodium carbonate
n(Na₂CO₃ ) = 0.5 M x 0.025 = 0.0125 mol

and from the chemical equation we find that
n(HCl) = 2 x n(Na₂CO₃) = 2 x 0.125 = 0.250 mol

Now if we want to find the concentration of something we will rearrange the equation n = cv into c = n/v

and the current information that we have are the "moles" of HCl which is 0.250 mol and volume used for the titration 23.92 mL

Let us know if you need further assistance

[peachxmh]

Your mol values look good

So we first have the sodium carbonate
n(Na₂CO₃ ) = 0.5 M x 0.025 = 0.0125 mol

and from the chemical equation we find that
n(HCl) = 2 x n(Na₂CO₃) = 2 x 0.125 = 0.250 mol

Now if we want to find the concentration of something we will rearrange the equation n = cv into c = n/v

and the current information that we have are the "moles" of HCl which is 0.250 mol and volume used for the titration 23.92 mL

Let us know if you need further assistance

Still a little confused ahaha

- So I've also figured out concentration of HCl in the titre by doing c = 0.250/0.02392 which is 10.451M
- Then I tried to use the values of the cleaner before and after the dilution to find the original concentration of HCl in the concrete cleaner by using C₁V₁ = C₂V₂ formula and substituting V₁ for 25.00 (the original undiluted volume of the cleaner), C₂ for 10.451 (the concentration of the HCl in the titre I found above) and V₂ for 250.0 (the diluted volume of the cleaner).
- After solving this equation, I got 104.51M which is way off the correct answer.

Where have I gone wrong?

[Sine]

Still a little confused ahaha

- So I've also figured out concentration of HCl in the titre by doing c = 0.0250/0.02392 which is 10.451M This is actually 1.04515
- Then I tried to use the values of the cleaner before and after the dilution to find the original concentration of HCl in the concrete cleaner by using C₁V₁ = C₂V₂ formula and substituting V₁ for 25.00 (the original undiluted volume of the cleaner), C₂ for 10.451 (the concentration of the HCl in the titre I found above) and V₂ for 250.0 (the diluted volume of the cleaner).
- After solving this equation, I got 104.51M which is way off the correct answer. So now your final answer should be 10.4515 M

Where have I gone wrong?

^ amendments made in red

I think the answer in your book may be incorrect.

If you work backwards form the answer 9.032M you get this which doesn't correspond with the info given
you get 0.9032M for aliquot
n = cv = 0.9032 x 0.02392 = 0.0216 mol

[Alohaaaa]

Hey guys,

I will be taking chemistry next year so I thought it’d be a good idea for me to go through a little bit of the course myself. Although I’m having trouble understanding elements. They are often defined as substances made up of one type of atom but what exactly are substances? If someone could explain this to me in simple terms I would be very appreciative.

Thanks in advance!

[Bri MT]

Hey guys,

I will be taking chemistry next year so I thought it’d be a good idea for me to go through a little bit of the course myself. Although I’m having trouble understanding elements. They are often defined as substances made up of one type of atom but what exactly are substances? If someone could explain this to me in simple terms I would be very appreciative.

Thanks in advance!

"substance" basically just means "thing"

You can think of atoms as like tiny lego pieces that make up the word. With lego pieces, we might group them into sizes based on their dimensions eg. 2 by 3, 2 by 2. With atoms we group them into elements based on how many protons they have eg. hydrogen (1 proton) helium (2 protons).


With lego pieces we can also describe their different colours, but this is less important for how they actually function and work together to make a structure and doesn't change their size.
Kind of similarly, we can also describe how many neutrons an atom has, but hat's not as important and doesn't change what type of element that atom is.

[Mattjbr2]

Does the delta H value in the diagram always represent a balanced complete combustion equation with integer coefficients?
Or can it sometimes represent a single mole of fuel being combusted?
In the data book, the delta H values represent a single mole of reactant fuel. 
I think I've seen questions where the graph represented only a single mole being reacted...unless I'm wrong.

[sweetcheeks]

Does the delta H value in the diagram always represent a balanced complete combustion equation with integer coefficients?
Or can it sometimes represent a single mole of fuel being combusted?
In the data book, the delta H values represent a single mole of reactant fuel. 
I think I've seen questions where the graph represented only a single mole being reacted...unless I'm wrong.

For heats of combustion, it should always be one mole of the substance being burnt (hence where kJ/mol comes into it, it is the energy in kilojoules when one mole is being combusted). The question you have shown is terrible, as I would always expect a combustion graph to be only considering 1 mole of fuel being burnt, regardless of the stoichiometric value of the oxygen.

[Mattjbr2]

For heats of combustion, it should always be one mole of the substance being burnt (hence where kJ/mol comes into it, it is the energy in kilojoules when one mole is being combusted). The question you have shown is terrible, as I would always expect a combustion graph to be only considering 1 mole of fuel being burnt, regardless of the stoichiometric value of the oxygen.

Thanks

I have another question.
In the older data book (pre-June 2018), how did the lactose molecule represent a glucose and a galactose?

The part on the left was a hexose with both hydroxyls pointed up => beta galactose
The part on the right was a hexose with the hydroxyl on the 4th carbon pointing up and the hydroxyl on the first carbon pointing down => alpha galactose
A glucose should have the hydroxyl on the 4th carbon pointing down

Right?

[sweetcheeks]

Thanks

I have another question.
In the older data book (pre-June 2018), how did the lactose molecule represent a glucose and a galactose?

The part on the left was a hexose with both hydroxyls pointed up => beta galactose
The part on the right was a hexose with the hydroxyl on the 4th carbon pointing up and the hydroxyl on the first carbon pointing down => alpha galactose
A glucose should have the hydroxyl on the 4th carbon pointing down

Right?

Correct. VCAA aren't the greatest at checking the molecular structures of their molecules (See 2016 aspirin).

[Mattjbr2]

In regards to NMR: Why does the nucleus align itself against the external magnetic field when you give it some radio waves? That makes no sense to me. Why wouldn't it just stay in the stable position?
The textbook doesn't explain why; neither do any of the videos/websites I've looked at.

[sweetcheeks]

In regards to NMR: Why does the nucleus align itself against the external magnetic field when you give it some radio waves? That makes no sense to me. Why wouldn't it just stay in the stable position?
The textbook doesn't explain why; neither do any of the videos/websites I've looked at.

Why does an electron get promoted to a higher energy level when light is shined on it? It's the same principle. Why would an electron become less stable? The particles don't really have a choice. if the correct wavelength (in this case in the radio wave frequency) is passed through a nucleus, the proton (and neutron) will absorb this energy and be promoted to the less stable level (in this case spin against). The proton can go back down to it's lower energy state and release the energy it absorbed (much like electrons moving down to lower energy levels after becoming excited).

[Mattjbr2]

Why does an electron get promoted to a higher energy level when light is shined on it? It's the same principle. Why would an electron become less stable? The particles don't really have a choice. if the correct wavelength (in this case in the radio wave frequency) is passed through a nucleus, the proton (and neutron) will absorb this energy and be promoted to the less stable level (in this case spin against). The proton can go back down to it's lower energy state and release the energy it absorbed (much like electrons moving down to lower energy levels after becoming excited).

Why doesnt it have a choice? Why not absorb the energy and stay parallel to the external field? I want to know why the nucleus flips anti-parallel to an external magnetic field (obtains a beta spin state) when radio waves are absorbed. Why not just stay parallel to the external magnetic field? The textbook, scishow, khanacadwmy ans chemguide all same the same thing: the nucleus needs energy to flip against the external magnetic field. They just never tell us why this is the case. I tried reading wikipedia but its so ridiculously dense, i couldnt understand anything.

[Bri MT]

Thanks for the response, but i wasn't talking about the excitation of electrons! I want to know why the nucleus flips anti-parallel to an external magnetic field (squares a beta spin state) when radio waves are absorbed. Why not just stay parallel to the external magnetic field? The textbook, scishow, khanacadwmy ans chemguide all same the same thing: the nucleus needs energy to flip against the external magnetic field. They just never tell us why this is the case. I tried reading wikipedia but its so ridiculously dense, i couldnt understand anything.

Sweetcheeks was using the excitation of electrons as a comparison to explain the nucleus flipping - might be worth a reread