Chemistry 3/4 2013 Thread

[clıppy]

Ooooh okay yeah so its direct and logical from equation to equation. Thanks

[Bhootnike]

the process below is only 40% efficient, determine the mass of HF required to produce 15.0 g of SiF4.
Si02 +4HF -> SiF4 + 2h20

wtf do i do ? 

[Ancora_Imparo]

First work out the mass of HF required to produce 15.0 g of SiF₄ if the process was 100% efficient.
n(SiF₄) = m/M = 15.0/M(SiF₄)
n(HF if 100% efficient) = 4*n(SiF₄)
m(HF if 100% efficient) = n*M = n(HF if 100% efficient)*M(HF)

Then, since it's 40% efficient, simply divide the mass determined for 100% efficiency by 0.4.
m(HF if 40% efficient) = m(HF if 100% efficient)/0.4

[justsoundslikeaworn-outcliche]

Hey I just need help with these 2 questions, and direction would be good cheers in advance!

[Daenerys Targaryen]

bi) you know the mass of the Ag2CrO4 as 0.788
Divide that my its molar mass to find the mol, relate it to the mol of K2CrO4
which is equal to the mol of
Times the molar mass of that with the mol

ii) relate the mol of Ag2CrO4 to that of the AgNO3 and times by molar mass

[justsoundslikeaworn-outcliche]

but this bi) and bii) is to find the mass not the mol??
So sonfused ;?

[Daenerys Targaryen]

multiplying the mol by the Molar mass gives the mass

[justsoundslikeaworn-outcliche]

bi) you know the mass of the Ag2CrO4 as 0.788
Divide that my its molar mass to find the mol, relate it to the mol of K2CrO4
which is equal to the mol of
Times the molar mass of that with the mol

ii) relate the mol of Ag2CrO4 to that of the AgNO3 and times by molar mass

So I'd just proceed by going: 0.788/331.73 gmol=0.0024mol of Ag2CO3
which equals the mol in K2CrO4
so mass of dicromate= 0.0024X dicromate molar mass (294.185) = 0.706044g?

[Daenerys Targaryen]

yeah sounds about right

[justsoundslikeaworn-outcliche]

yeah sounds about right

about right thats reassuring

[Daenerys Targaryen]

that is right?

[Jeggz]

about right thats reassuring

It is correct 

[Daenerys Targaryen]

It is correct 

Eugh! Shoul've added a question mark at the end. Would have many it ambiguous and scary. It would've been a mystery

[clıppy]

I've got a SAC tomorrow so hopefully someone can help me with this gravimetric question.
I'm not sure if its necessary to know but we used BaCl₂ to carry out our precipitation reaction. We are using gravimetric analysis to find the sulphur content in fertiliser and one of the questions is:

This fertiliser is an example of an NPK fertiliser which contains Nitrogen, Phosphorus and Potassium. These elements are typically in the form of (NH₄)₂SO₄ and Ca₂(H₂PO₄) and K₂SO₄ respectively. Could any other elements in this fertiliser be analysed by gravimetric analysis. If so, what solution would be added to carry out the precipitation?

Is the question asking me to find in relation to using barium chloride or is it asking for me to add something else to make a new precipitate to determine one of the other elements?

Also in relation to the discussion (about what my errors could've been) if my percentage was lower i could say that when the barium chloride is added to form the precipitate, not allowing for proper cooling could've hindered the precipitation because solubility is higher at higher temperatures.
I'm not exactly sure how that works, could someone explain it to me please?

[MonsieurHulot]

Hey everyone, I've just got a couple of questions about a prac we're doing to determine the content of ethanol and sulfur dioxide in wine.

Why may the SO2 value differ from what is stated on the packaging?

We had to add 2M H2SO4 to a solution that had had NaOH added to it, why was it necessary to add the H2SO4 to only one flask, titrate, and then move on to the next?

Thank you.