Help with Chemistry Calculations (1 Viewer)

[misspandoraa]

Hello Everyone!

I was just wondering if there are some good advice or some summaries that go through every possible TYPE of calculation they might ask for HSC, Trials or chem practicals. I'm confident with theory and the non calculation parts of chemistry, but failing 10% of my chemistry course over calculations suck. (I'm terrible at maths)

So yes, I know really basic formulae like n=cv n=m/M etc.
It's just applying them, especially in titrations and dilutions, and things in excess or whatnot that completely screw over my mind.

Any help?
Thanks in advance

 

[RANK 1]

give an example question that you cant do and i'll try to explain it to u

 

[adz1098]

i have a questrion, when a calc involves limting reagants, you use inital reacting excess do you use the amount of moles in exess for the rest of the calulation or the amoiunt of moles reacted

 

[Alkanes]

You use the value of the limiting reagent because they will always run out first.

 

[adz1098]

so the the one which is all used up, how come in back titrations and some Ph questions you use the excees value of mols?

 

[UnrealAnchovies]

i have a questrion, when a calc involves limting reagants, you use inital reacting excess do you use the amount of moles in exess for the rest of the calulation or the amoiunt of moles reacted

wut?

for limiting reagents you do the straightforward moles work with the data you've been provided. this is how much of a substance you HAVE.

then you play around with mole ratios to find the amount you REQUIRE. you look at these and then can determine the limiting reagent.

all calculations which follow will be according to the amount of limiting reagent (unless it specifies something to do with excess). i really can't explain over the net tbh. try posting up a question.

 

[UnrealAnchovies]

so the the one which is all used up, how come in back titrations and some Ph questions you use the excees value of mols?

because you simply need to find pH. think about it this way. you got 20 grams of strongly basic sodium hydroxide and 500 grams of hydrochloric acid (im using grams for simplicity's sake, it really should be in moles), when all the sodium hydroxide has reacted, you've got excess hydrochloric acid. it hasn't been neutralised. it's just there, and will therefore lower the pH.

 

[adz1098]

A chemist wanted to find the percentage of caco3 in 2g of limewater. He added 100 ml of 0.905 mol/l HCL which was more than enough to react with with the calcium carbaonate. After the bubbling had ceased, the resulting solution was titrated with sodium carbonate solution. 30.45 ml of 1.01 mol/l was required to react with the unreacted HCL.

 

[RANK 1]

you sure it was 2g of limewater? cos i got 2.99g of caco3?