HSC Chemistry Question Thread

[mcheema]

Hi,
Ive noticed that majority of the exams and quizzes I do I mainly get half equations wrong.
Can you please provide some tips and explain how to do half equations
Thanks

[Kekemato_BAP]

Hi,
Ive noticed that majority of the exams and quizzes I do I mainly get half equations wrong.
Can you please provide some tips and explain how to do half equations
Thanks

Hi, this is how I do all my half equations and net ionics. We'll use calcium with hydrochloric acid as an example:

1: Write the equation first. Ca + 2HCl -> CaCl2 + H2
2: Write the oxidation equation for the calcium metal. Ca -> Ca2+ + 2e-
3: Write the reduction equation for the hydrogen ions from the HCl. 2H+ + 2e- -> H2
4: The net ionic equation is basically the two half equations combined but minus the electrons. Ca + 2H+ -> Ca2+ + H2

That's how I do it.

[anotherworld2b]

I just had another read of my assignment. I'm not sure what calculations could be used to determine tartaric acid concentration in wine using this method to be honest. I'm sorry for asking so many questions. I don't really understand titrations very well.

Yep, your method will do the trick! To find the acidity of a substance, you need to titrate it. You also need to know the chemical making the wine acidic, so that you can write out the chemical formula. From there, you carry out the usual mathematical steps to find the concentration (and therefore pH) of the wine.

As for your second question, you haven't really explained WHY it is placed in an oven; merely how it is heated up/kept dry etc. Obviously, you know the answer; it is to ensure the substance doesn't absorb water, affecting the purity and the way in which you measure to mass/moles used to create a standard solution. However, you haven't directly answered the question.

[mcheema]

Hi, this is how I do all my half equations and net ionics. We'll use calcium with hydrochloric acid as an example:

1: Write the equation first. Ca + 2HCl -> CaCl2 + H2
2: Write the oxidation equation for the calcium metal. Ca -> Ca2+ + 2e-
3: Write the reduction equation for the hydrogen ions from the HCl. 2H+ + 2e- -> H2
4: The net ionic equation is basically the two half equations combined but minus the electrons. Ca + 2H+ -> Ca2+ + H2

That's how I do it.

Thanks for the help. Yes that makes more sense now

[jakesilove]

I just had another read of my assignment. I'm not sure what calculations could be used to determine tartaric acid concentration in wine using this method to be honest. I'm sorry for asking so many questions. I don't really understand titrations very well.

Hey!

So, we're titrating Tartaric acid and Sodium hydroxide. We have the concentration (and thus moles) of Sodium hydroxide; and we can find the volume (and thus concentration) of Tartaric acid using the titration process. First, we need a chemical equation.



I had to google this. From here, we can do straight forward titration calculations to find the concentration of acid. Find the number of moles of NaOH used, and halve it to get the number of moles of Tartaric acid. Finally, divide the moles of acid by the volume used to get the concentration.

[anotherworld2b]

thank you for your help
I was also wondering how do you choose the correct indicator for a tit ration involving sodium hydroxide and hydrochloric acid?
methyl orange or bromophenol blue?

Hey!

So, we're titrating Tartaric acid and Sodium hydroxide. We have the concentration (and thus moles) of Sodium hydroxide; and we can find the volume (and thus concentration) of Tartaric acid using the titration process. First, we need a chemical equation.



I had to google this. From here, we can do straight forward titration calculations to find the concentration of acid. Find the number of moles of NaOH used, and halve it to get the number of moles of Tartaric acid. Finally, divide the moles of acid by the volume used to get the concentration.

[jakesilove]

thank you for your help
I was also wondering how do you choose the correct indicator for a tit ration involving sodium hydroxide and hydrochloric acid?
methyl orange or bromophenol blue?

Strong acid + strong base will have an end point that's about neutral. So, we use Bromothymol Blue

[anotherworld2b]

I was wondering how do you do this question?

[armtistic]

I was wondering how do you do this question?

You already found the moles of CaO and now you're finding them again in your last step when you should be using m = n x mm to find mass

[anotherworld2b]

I see now what I did wrong.
Thank you very much for your help

You already found the moles of CaO and now you're finding them again in your last step when you should be using m = n x mm to find mass

[anotherworld2b]

I was trying to solve this question but I'm not sure what to do. Can i have some help please?

[bsdfjnlkasn]

Hey there,

I was wondering if someone could help me with the calculations in the following question. Specifically the "alternate calibration method" where you don't use the calibration curve. I haven't been formally taught this yet and the textbook explanation is a bit confusing so any help would be greatly appreciated 

EDIT: I also have another question - Why does afternoon peak hour traffic have much less effect on city ozone concentrations than the morning peak?

From my textbook, I seem to be under the impression that the ozone concentration is higher in the afternoon because over the day, the NO_x which are initially emitted by car exhausts in the morning peak, are oxidised and then photodissociated into free radical which then react with O₂. Am I missing a step where the ozone just randomly disappears because that seems to be the only missing piece that would allow me to properly explain what the question implies.

[jakesilove]

I was trying to solve this question but I'm not sure what to do. Can i have some help please?

This is just a standard titration question. Find the chemical equation. Find the number of moles of the known substance. Use the chemical equation to divide/multiply this value to get the moles of the unknown. Then, divide my the volume to get the concentration

[jakesilove]

Hey there,

I was wondering if someone could help me with the calculations in the following question. Specifically the "alternate calibration method" where you don't use the calibration curve. I haven't been formally taught this yet and the textbook explanation is a bit confusing so any help would be greatly appreciated 

EDIT: I also have another question - Why does afternoon peak hour traffic have much less effect on city ozone concentrations than the morning peak?

From my textbook, I seem to be under the impression that the ozone concentration is higher in the afternoon because over the day, the NO_x which are initially emitted by car exhausts in the morning peak, are oxidised and then photodissociated into free radical which then react with O₂. Am I missing a step where the ozone just randomly disappears because that seems to be the only missing piece that would allow me to properly explain what the question implies.

Hey! I'm not sure what you mean by the 'alternate calibration' method; as far as I can tell, you need to sketch a calibration curve using the standard solutions (ie Absorbance vs. ppm) and then find on that graph where the absorbance is 0.32, 0.46, 0.21 and 0.055 to find the ppm value of each of the solutions A,B,C and D. Does that kind of make sense? First, you'll need to convert each standard value into ppm. Then, plot it against absorbance and join a line of best fit.

As for your second question, that's way, way outside of the curriculum and is definitely something that you DO NOT need to know. I had no idea that ozone concentrations were higher in the afternoon. It is important to note that Ozone can be formed by combustion in car exhausts, but beyond that you seriously will never need to know any of the information you've described above.

[anotherworld2b]

Like this?

This is just a standard titration question. Find the chemical equation. Find the number of moles of the known substance. Use the chemical equation to divide/multiply this value to get the moles of the unknown. Then, divide my the volume to get the concentration