Homework questions thread

[Tonychet2]

i have no idea then but i wanna find out the answer now too lol

wait did u type the qusetion up properly?

[charmanderp]

I'm pretty sure it's asking for a percentage by mass as the final answer, rather than concentration. So you'd use the concentration of the solution to figure out how much the cadmium in the 10ml solution weighs, divide it by the mass of the potato and then convert it to ppb. I got 78 (sig figs).

PPB is equal to micrograms per litre. So 3.5ppb is equal to 3.5*10^-6 grams per litre.

So multiply it be the volume of the solution to get 3.5*10^-8 grams of cadmium in the potato.

Divide this by the mass of the potato and multiply by 100 to get percentage by mass. Multiply that by 10,000 to get ppm. And then multiply that by 1000 to get ppb. Should give you 77.777777777777778 but our concentration in ppb was given to two sig figs, so our answer should be too. 78ppb.

[destain]

Well yeah that would be the right answer, their working out is
0.035ug in 10ml
thus 0.035ug in 0.45g of the potato sample.
0.035 x 1000/0.45ug in 1000g
=77.78 ppb...
can you explain that and how are you meant to know what its asking for? is it because it says concentration in the potato not in a solution?

[charmanderp]

Well obviously if you have a potato and you have a solution, it's referring to different forms of concentration, as one is a solid and the other is a liquid. Concentration is a ratio either by mass or volume of a substance to the mass or volume of whatever it is in.

It's a poorly worded question but I guess you have to make heads or tails of it.

[Tonychet2]

wait nevermind im really stupid lol

3.5ppb = 3.5ug/L
u have 10 ml so its 0.035 ug (3.5*10^-8) in 0.45g

divide by 0.45 so u get 7.78^10-8 grams per 1 gram

multiply by 10^9 to get parts per billion which is 77.78

[Tonychet2]

Well obviously if you have a potato and you have a solution, it's referring to different forms of concentration, as one is a solid and the other is a liquid. Concentration is a ratio either by mass or volume of a substance to the mass or volume of whatever it is in.

It's a poorly worded question but I guess you have to make heads or tails of it.

thats wat tricked me lol nice work! u will get a 50 believe me lol

[destain]

ugh ok.. so basically percentage by pass in ppb... once you get the grams, you can just multiply by one billion for 77.78..?

and also whats the actual difference between reagent and catalyst..
E.g. From ethene to ethanol, what is the reagent
and from Ethanol to Ethanoic acid
Ethanoic acid and propanol to Proply ethanoate...

[Tonychet2]

reagents get used up in the reaction and catalysts don't

and the method to ppb is get the two numbers in the same form so grams in terms of grams then multiply by a billion (x10^9, not x10^8 sorry i made a mistake in my last post) to get ppb

[destain]

cause i said
H3PO4 for ethene to ethanol...but the answer is H2O?
and is Cr2O72- a reagent?
also H2SO4 for the esterification reaction.

[charmanderp]

ugh ok.. so basically percentage by pass in ppb... once you get the grams, you can just multiply by one billion for 77.78..?

Yes! I just like to do it step by step because numbers confuse me haha.

[Tonychet2]

cause i said
H3PO4 for ethene to ethanol...but the answer is H2O?
and is Cr2O72- a reagent?
also H2SO4 for the esterification reaction.

h2o is the reagent because its used up in the reaction
cr2o72 i think is a catalyst? but 2 oxygen molecules come from somewhere im not sure on this one
h2so4 is a catalyst

[charmanderp]

Dichromate is the reagent, the acid added is the catalyst.

A catalyst is something which speeds up the rate of a reaction whilst itself having no impact on the physical state of the product. A reagent is something which as added to another substance to cause a physical change (or something along these lines).

In order to hydrate ethene to convert it into ethane, you have to add water (obviously), which is the reacting agent - it causes the reaction. So that the reaction doesn't take forever, you add H3PO4.

Conversion of a primary alcohol to a carboxylic acid is called oxidation. So you need an oxidant, and dichromate happens to be a very good one. Once again, it is a reacting agent. We add H+ ions as well so that the reaction between oxidant and reductant happens sooner rather than later.

In esterification we have two reactants, an alcohol and an acid. This is enough to form our final product, so we don't need any more reagents. However without a catalyst the reaction would take thousands of years to happen, so we need to add sulfuric acid in liquid form. EDIT: So in a way, the catalyst is here a reagent, but it's not consumed in the reaction like H2O was in hydration, and it doesn't change form like dichromate does in redox.

[destain]

so sulfuric acid is a catalyst not a reagent? and therefore the exam is wrong?
but the solutions is so weird it says
Reagent C (sulfuric acid) is a catalyst needed to blablahblahbabha...
so its calling it a reagent yet a catalyst?

[charmanderp]

Well it's both a reagent and a catalyst, I guess. But it's not a reagent in the sense that the others are.

[destain]

ah okay thanks, i see just another confusing question...
how about this one

Which of the following could undergo oxidation to a carboxylic acid when treated with an approprite oxideant
CH3CH2CHO
CH3CH(OH)CH3
CH3COOH
(CH3)3COH

answer is A and i dont understand how taht works, i chose D,