HSC Chemistry Question Thread

[massive]

Can someone please do this the answer is 12.56

[jakesilove]

Anyone have an idea?

Hey! Figure out the moles of Lead chloride using the final mass. The moles of Lead chloride will equal the moles of Lead, so we can just plug this value for 'n' into C=n/v (using a volume of 50mL, the initial volume) to get a concentration out!

[kimmie]

How do we measure biological oxygen demand?

[massive]

Hope this makes sense
(Image removed from quote.)
http://imgur.com/Fc4ehZQ

Damn thanks so much! can you have a look at the other q i posted as well please 

[noonedoesnt]

Hey! Figure out the moles of Lead chloride using the final mass. The moles of Lead chloride will equal the moles of Lead, so we can just plug this value for 'n' into C=n/v (using a volume of 50mL, the initial volume) to get a concentration out!

the only part i don't understand is why we use the 50 not the 25mL

[jakesilove]

Can someone please do this the answer is 12.56

We know that the molar ratio is 1:2 (one mole of Base requires two moles of Acid).







If we use 0.01 moles of acid, that will take up 0.005 moles of base, leaving us with 0.00355 moles of base left. The concentration of Hydroxide is therefore



Now, pH is 14+pOH, so 1-log(0.0182)=12.26. I think I probably messed up some calculation error somewhere, but that's the general method, so you can go back over and check each step so you can understand how to answer a question like this!

[jakesilove]

the only part i don't understand is why we use the 50 not the 25mL

Because the lead collected was as a result of 50mL of the pond sample, and we're interested in the lead concentration in the pond

[jakesilove]

How do we measure biological oxygen demand?

Above is a super condensed version of what you need to know. You probably don't have time to go over the section in depth, and understand it properly, so just throw in some of the buzzwords I've outlined and you can still get all the marks

[WLalex]

Nup, turns out that acetic acid is stronger than citric acid. That's just a fact, memorise it!

buuuut...does not acetic acid only ionise around 1% whereas citric is around 8% (would that not mean citric is a stronger acid)??

[jakesilove]

buuuut...does not acetic acid only ionise around 1% whereas citric is around 8% (would that not mean citric is a stronger acid)??

Yep, you're definitely right about that actually. Sorry, I don't remember what the original question was or why I wrote what I did; presumably, the thing to get out of this is that in terms of ionisation, HCl ionises the most, then citric, then acetic acid.

[massive]

We know that the molar ratio is 1:2 (one mole of Base requires two moles of Acid).







If we use 0.01 moles of acid, that will take up 0.005 moles of base, leaving us with 0.00355 moles of base left. The concentration of Hydroxide is therefore



Now, pH is 14+pOH, so 1-log(0.0182)=12.26. I think I probably messed up some calculation error somewhere, but that's the general method, so you can go back over and check each step so you can understand how to answer a question like this!

Thanks for this, does the fact that calcium hydroxide releases 2 OH make a difference?? :S

[jakesilove]

Thanks for this, does the fact that calcium hydroxide releases 2 OH make a difference?? :S

Yeah, see my initial line (knowing that it is a 1:2 reactions allows me to find the limiting reagent)

[kimmie]

How does electroplating work?

[jakesilove]

How does electroplating work?

Hey!

Electroplating works by setting up an electrolytic cell, with an anode and a cathode. Say we wanted to coat Iron is Silver. We would make Iron the Cathode, and Silver at the Anode. We would immerse both in a solution of Silver nitrate. The anode oxidises, thus producing more Silver ions. The cathode reduces, converting silver ions into silver metal, which plates the cathode. So, just set up the cathode with whatever you want to coat, set up the anode and the electrolyte with whatever you want the cathode to be coated in, and pump through some current!

[ProfLayton2000]

How does including and outlier affect the validity, reliability and accuracy of results?
(Specifically HSC 2004 q20)