STOICHIOMETRY- whoever gets it plz help-urgent!!!!gag (1 Viewer)

[Dreamerish*~]

ook, wud u kno of any other minor formulas tht are applicable

Hmm, depends on what topic you're talking about. Listing every formula used in preliminary chemistry would take me awhile.

You're familiar with ΔH = MCΔT?

 

[n a + a l i e]

yeh im kinda having probelms with that, firstly, tht deals with endo/exothermic reactions, the ultimate figure is negatve when its end, positive when endo,
is the formula as easy as the n=m/M formula

 

[Dreamerish*~]

yeh im kinda having probelms with that, firstly, tht deals with endo/exothermic reactions, the ultimate figure is negatve when its end, positive when endo,
is the formula as easy as the n=m/M formula

Yes, it's basically substitution.

ΔH = energy (J/kJ)
M = mass of water (g/kg)
C = heat capacity of water = 4.18 or 4.2, depending on source
ΔT = change in temperature (°C)

 

[n a + a l i e]

ur in yr 12 right? is there a lot of stoichiometry in yr 12 for chem

 

[Dreamerish*~]

ur in yr 12 right? is there a lot of stoichiometry in yr 12 for chem

Yes I am.

You need to remember all the concepts and basics of the preliminary course because that is the foundation of HSC chemistry. HSC topics are very different and sometimes irrelevant to what you are doing now, but everything you've learnt in year 11 becomes assumed knowledge in year 12.

 

[Dreamerish*~]

hey n also could you help me with limiting reagent problems

Sure, post one up.

 

[n a + a l i e]

100g of HCL(aq) is added to 100g of Zn. Find the volume of hydrogen gas evoloved under standard lab conditions. also how do u kno when is something an acid? irrelevant to first q but meh

 

[Dreamerish*~]

100g of HCL(aq) is added to 100g of Zn. Find the volume of hydrogen gas evoloved under standard lab conditions. also how do u kno when is something an acid? irrelevant to first q but meh

First of all, acids contain hydrogen. That explanation is probably too simple because bases contain hydrogen too (in the hydroxy group). By definition, an acid is a substance which in solution produces hydrogen ions, H⁺ or more correctly, H₃O⁺; and a base is a substance which either contains the oxide O^2- or hydroxide OH^- ion or which in solution produces the hydroxide ion.

You need to be familiar with common acids and bases:

Acids: hydrochloric (HCl), sulfuric (H₂SO₄), nitric (HNO₃), acetic or ethanoic (CH₃COOH), hydrobromic (HBr) and phosphuric (H₃PO₄). I might have missed one or two.

Bases: sodium hydroxide (NaOH), potassium hydroxide (KOH), basically metallic oxides and hydroxides. An alkali is a soluble base. Not all bases are alkalis. You don't need this just yet.

Now for your question, the balanced equation is:

2HCl_(aq) + Zn_(s) → ZnCl_2(aq) + H_2(aq)

We have 100 g of HCl and 100 g of Zn.

M of HCl = 1.008 + 35.45 = 36.458
M of Zn = 65.4

n of HCl = 100/36.458 = 2.74288
n of Zn = 100/65.4 = 1.52905

From the equation, the ratio of HCl to Zn is 2 : 1.
1.52905 (n of Zn) x 2 = 3.05810 (n of HCl required to react with this n of Zn), which is larger than the amount of HCl we have, therefore Zn is in excess.

The ratio of HCl : Zn : H₂ is 2.74288 : 1.37144 : 1.37144.

Therefore 1.37144 moles of H₂ is given off.

V = n x 24.79 (standard lab conditions is 25°)
= 34.00 L (to 2 d.p.)

 

[n a + a l i e]

also for acids would that just be.. anything with H in front and O at the back, its just a trend i noticed..? eg sulfuric acid

 

[DraconisV]

Hey na+alie can you please put up some more limiting questions.
And on the previous page there was a question about gases, volumes and empirical formula, can someone please check to see that it has been done correctly and can some one please post up a question that is similar to that hydrocarbon (70% blah, 30% bling) one.

Thx guys

DraconisV

 

[Dreamerish*~]

also for acids would that just be.. anything with H in front and O at the back, its just a trend i noticed..? eg sulfuric acid

No, acetic acid (CH₃COOH) - all alkanoic acids, infact; citric acid (C₆H₈O₇); hydrochloric acid (HCl); hydrobromic acid (HBr) - just to name a few - don't follow that trend.

Later you'll learn that acids are proton donors and bases are proton acceptors. However, neither does that provide you with a simple rule on differentiating between acids and bases.

You probably haven't come across all acids required in the HSC course yet. Make sure you know your preliminary stuff well - it's not too difficult to remember a few acids and bases, because learning about them requires you to remember both the formula and the name, and names of acids usually end in "acid".

 

[Dreamerish*~]

Hey na+alie can you please put up some more limiting questions.

1. What mass of magnesium hydroxide precipitates when excess sodium hydroxide is added to 50 mL of 0.2 M magnesium sulfate solution?
   
2. What mass of silver carbonate precipitates when 25 mL of 0.2 M silver nitrate solution is added to 30 mL of 0.150 M sodium carbonate solution?
   
3. What volume of 0.5 M hydrochloric acid reacts exactly with 3.0 g of sodium carbonate?
   
4. What mass of sulfur is produced in the reaction:
   2H₂S_(g) + SO_2(g) → 2H₂O_(l) + 3S_(s)
   ... when 4 L of H₂S is reacted with 3 L of SO₂, both volumes measured at 25°C and 101.3 kPa?
   
5. Work out the empirical formulae of the following compounds which contain:
   (a) 52.12% carbon, 13.13% hydrogen, 34.75% oxygen
   (b) 85.52% carbon, 14.38% hydrogen
   (c) 39.98% carbon, 6.72% hydrogen, 53.30% oxygen

 

[n a + a l i e]

1. 2Na(OH) + Mg(SO4)*Mg (OH)2+ Na2(SO4)
5Oml= mass <for liquids ml=g right?>
0.2M=no of moles <is that per L>
Mole ratio of Mg(SO4) : Mg (OH)2 = 1:1
therefore mass of Mg (OH)2= o.2M x (24+ 2(16+1))
=11.6 ml

thts the first one tell mi if its right because q 2 is pretty similar thnx

 

[Templar]

0.2M means concentration ie 0.2 mole/L.

50mL at 0.2M contains 0.2*0.05 moles in that solution.

 

[n a + a l i e]

ook then how do u kno when theyre telling u no o moles then?

 

[n a + a l i e]

solution for q1 of the qs u posted earler
1. 2Na(OH) + Mg(SO4)*Mg (OH)2+ Na2(SO4)
5Oml= volume
0.2M=n/50ml
N=50 x .2=10
Mole ratio of Mg(SO4) : Mg (OH)2 = 1:1
therefore mass of Mg (OH)2= 10 x (24+ 2(16+1))
=580g

 

[Templar]

550mg since you divided concentration in M by mL to obtain how many millimoles. Remember to convert everything into standard units.

 

[n a + a l i e]

2 Qs. Do moelcular compounds only display molecular formula and ionic compounds display empirical formula, and what about covalent compounds
How do you work out empirical formula when you are given precentage compositions of the compounds especially for carbon compounds u can refer to the qs posted bi dreamerish.

 

[Dreamerish*~]

2 Qs. Do moelcular compounds only display molecular formula and ionic compounds display empirical formula, and what about covalent compounds
How do you work out empirical formula when you are given precentage compositions of the compounds especially for carbon compounds u can refer to the qs posted bi dreamerish.

I'll do one of them for you:

Work out the empirical formulae of the following compounds which contain:
(a) 52.12% carbon, 13.13% hydrogen, 34.75% oxygen

M carbon = 12 g
M hydrogen = 1.008 g
M oxygen = 16 g

52.12/12 = 4.34
13.13/1.008 = 13.03
34.75/16 = 2.17

We have a ratio of C : H : O of - 4.34 : 13.03 : 2.17
Dividing by the smallest number (2.17), we get - 2 : 6 : 1

Therefore our empirical formula is C₂H₆O, which can also be written as C₂H₅OH - ethanol.

My reasoning behind doing these questions is to work backwards, for example, if you have the compound C₃H₈, and you want to work out the percentage composition of carbon and hydrogen, what do you do?

First you calculate the total mass - 12 x 3 + 1.008 x 8 = 44.064

Now, the mass of carbon in propane is 12 x 3 = 36 and the mass of hydrogen in propane is 1.008 x 8 = 8.064.

Respectively, their percentages would be roughly 36/44.064 = 81.70% and 8.064/44.064 = 18.30%.

For the empirical formula questions, you basically work backwards.

 

[Dreamerish*~]

ook then how do u kno when theyre telling u no o moles then?

That's if they tell you "A reacted with exactly 0.0345 moles of B" which they probably will never do.

You're usually given the volume of reactants and their concentration, which is written as M or as mol L^-1. Be careful that you don't take these units as the number of moles - they're the concentration.