Post your chemistry questions here. (1 Viewer)

Alkanes · Sep 29, 2011

Ca + 2H2O -----> H2 + Ca(OH)2

n = 0.1 L x 3.16 x 10^-2
= 0.00316 moles

V(H2) = 0.00158 mol x 24.79 L/mol
= 0.0389 L

I think its right.

MagnificentOne · Oct 5, 2011

Q: A 5g tablet contains sodium carbonate. The tablet was crushed then reacted with 20mL, 0.1M HCl. The excess acid was titrated against 0.1M NaOH needing 5mL. Calculate % of sodium carbonate in the tablet

Free rep whoever comes with correct answer!

RANK 1 · Oct 5, 2011

MagnificentOne said:
Q: A 5g tablet contains sodium carbonate. The tablet was crushed then reacted with 20mL, 0.1M HCl. The excess acid was titrated against 0.1M NaOH needing 5mL. Calculate % of sodium carbonate in the tablet

Free rep whoever comes with correct answer!

1.58985%

2% (1 s.f)

MagnificentOne · Oct 6, 2011

NicE!

snorelacks · Oct 6, 2011

Still a bit confused with titration questions.
25.0mL of 0.12M standard barium hydroxide solution was titrated with nitric acid. The results are recorded as below
Titration 1 : 20.4mL
Titration 2 : 18.1mL
Titration 3 : 18.2mL
Titration 4 : 18.1mL

Calculate the concentration of nitric acid.
I basically understand the whole question, but still am confused as to when I have to add the two volumes of known solutions together? (for the last step to find the concentration)

RANK 1 · Oct 7, 2011

snorelacks said:
Still a bit confused with titration questions.
25.0mL of 0.12M standard barium hydroxide solution was titrated with nitric acid. The results are recorded as below
Titration 1 : 20.4mL
Titration 2 : 18.1mL
Titration 3 : 18.2mL
Titration 4 : 18.1mL

Calculate the concentration of nitric acid.
I basically understand the whole question, but still am confused as to when I have to add the two volumes of known solutions together? (for the last step to find the concentration)

excluding the rough titre, the average is 0.018333...L

Ba(OH)₂ + 2HNO₃ ---> 2H₂O + Ba(OH)₂

therefore C_Ba(OH)₂V_Ba(OH)₂=1/2C_HNO₃V_HNO₃ as the stoichiometry is 1:2

[0.12][O.O25] = [1/2]C[0.018333..]
[1/2]C = [0.12][0.025]/[0.018333..]
[1/2]C = 0.165441176
C = 0.330882352 mol/L
C = 0.331 mol/L (3 s.f)

RANK 1 · Oct 7, 2011

Alkanes said:
I think this one you need to disregard the first titration and take the average of the last 3. So firstly find the number of moles which you should get 0.003 moles.

C = 0.003 mol / [(18.1+18.2+18.1)/3/1000)]
= 5.4 x 10^5 mol/L

I think

its definitely not 5.4 x 10^5 mol/L

Alkanes · Oct 7, 2011

Ahh shit silly me.. Forgot to write an equation hence why the mole ratios fked up.. =(

So it would be C = 0.006 / 0.018333
= 0.330882 mol/L
= 0.33 mol/L (2sf)

Also wouldn't it be 2 sf not 3?

RANK 1 · Oct 7, 2011

no apparently in chemistry xx.0 counts as three

Alkanes · Oct 7, 2011

Don't you take into account from 0.12 M ?

apollo1 · Oct 7, 2011

can someone explain to me the stupid significant figures thing in chemistry. pisses me off.

Also heres a question:

Screen shot 2011-10-07 at 4.46.16 PM.jpg

explanation plz with answer.

RANK 1 · Oct 7, 2011

Alkanes said:
Don't you take into account from 0.12 M ?

oh... right my bad

RANK 1 · Oct 7, 2011

apollo1 said:
can someone explain to me the stupid significant figures thing in chemistry. pisses me off.

Also heres a question:

View attachment 23145

explanation plz with answer.

its b cos the light gets absorbed by the atoms

Alkanes · Oct 7, 2011

apollo1 said:
can someone explain to me the stupid significant figures thing in chemistry. pisses me off.

Also heres a question:

View attachment 23145

explanation plz with answer.

Your final answer should only posses the lowest sig figs from the data. So for example you're given 0.1555 g (4sf) of magnesium and oxygen 15.1g 3 (sf). Find the mass of magnesium oxide. You answer should be only 3 sf. xx.x g or 0.xxx g

umz93 · Oct 7, 2011

Can someone please explain oxidation/reduction states to me.

Reikira · Oct 7, 2011

Q: Using TWO examples, analyse how the features of catchment areas will determine the water treatment necessary to make the water safe to drink.

RANK 1 · Oct 7, 2011

Reikira said:
Q: Using TWO examples, analyse how the features of catchment areas will determine the water treatment necessary to make the water safe to drink.

i did this question yesterday, ill answer it after diiner

RANK 1 · Oct 7, 2011

umz93 said:
Can someone please explain oxidation/reduction states to me.

ill do this after dinner as well

RANK 1 · Oct 7, 2011

umz93 said:
Can someone please explain oxidation/reduction states to me.

oxidation/reduction states refers to the charge an atom has or the number of extra or reduced electrons an atom has on the outer valence shell.

RANK 1 · Oct 7, 2011

Reikira said:
Q: Using TWO examples, analyse how the features of catchment areas will determine the water treatment necessary to make the water safe to drink.

'Features of catchment areas that determine if water treatment is necessary include proximity to farmland, sewage and mines. Farmlands and sewage areas release effluents into the water, causing the water to have to be treated before consuming. Bacteria, such as E.coli from feces also need to be treated before the water is safe to consume, chlorination would be a suitable treatment. Screening and filtering may also be necessary to remove particles from sewage run offs.

mines are a potential source of harmful heavy metals which could leak into the waterways. an example of a harmful heavy metal is lead, which is bioaccumulative, affects enzyme production, hemoglobin production in humans and can kill if the body absorbs too much lead. water near mines would need to be treated with ion selective membranes such as PTFE.'

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