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# Quantative OCR as level chemistry help watch

1. Could anyone answer these questions please please please!!! I need urgent help as I have a test coming up and I need to be shown how to answer these sort of questions as I'm never going to succeed if I can't learn from my mistakes.

1) magnesium oxide is made by the following reaction: Mg+CuO=MgO+Cu

I) determine the atom economy for the production of magnesium oxide using his reaction
Ii)what determines what reaction has the highest atom economy out of two reactions

2) red phosphorus reacts with hydrogen to produce toxic phosphine gas PH3. Calculate the percentage by mass of phosphorus in phosphine

I) phosphine has can also be made by reacting white phosphorus with water. The products are phosphine gas and phosphoric acid (H^3PO^4) construct an equation from this reaction

Ii) write an equation for the reaction of white phosphorus with oxygen to produce P^4O^6 include state symbols

3) suggest a step that could be added to a water of crystallisation reaction that would allow you to know when all of the water of crystallisation had been driven off

4) student followed method but noticed that when strongly heating their crystals some of them spat out of their crucible. Will this made their calculation of x too high or too low

5) a student attempted to calculate relative molecular mass of an unknown volatile liquid. 4.17g of the liquid was gently evaporated in an oven at 18C at a pressure of 101kPa. The gas occupied 2.5Odm^3. Calculate the relative molecular mass of gas
2. 1i) (desired product/ total mr of product)x1000
1ii) is there meant to be another reaction to look at?
3. 2) (molar mass of phosphorous/ molar mass of phosphine)x1000
2i) P + H2O => H3PO4 + PH3 (this isnt balanced-- if you didnt understand where this equation came from, it may be worth revising the gcse syllabus)

LHS(reactants) -> P-1 H-2 O-1
RHS(products) -> P-2 H-6 O-4

1) put a 2 in front of the P (2P + H2O => H3PO4 + PH3)
LHS(reactants) -> P-2 H-2 O-1
RHS(products) -> P-2 H-6 O-4
2) put a 3 in front of the H2O (2P + 3H2O => H3PO4 + PH3)
LHS(reactants) -> P-2 H-5 O-3
RHS(products) -> P-2 H-6 O-4
3) this is where it gets a bit difficult because you have to mess around with things you've already balanced 12 is the LCM of 3 and 4, so place a 4 in front of water (2P + 12H2O => H3PO4 + PH3)
LHS(reactants) -> P-2 H-24 O-12
RHS(products) -> P-2 H-6 O-4
4) then place a 3 in front of the salt (2P + 12H2O => 3H3PO4 + PH3)
LHS(reactants) -> P-2 H-24 O-12
RHS(products) -> P-3 H-9 O-12
5) now everything except the oxygens are unbalanced! luckily there is a compound which contains just the elements that are not balanced (PH3)
place a 5 in front of the phosphine gas (2P + 12H2O => 3H3PO4 + 5PH3)
LHS(reactants) -> P-2 H-24 O-12
RHS(products) -> P-8 H-24 O-12
6) now its just the phosphorous, so place a 4 in front of 2P (since 2 * x = 8, x = 4)
(8P + 12H2O => 3H3PO4 + 5PH3)
LHS(reactants) -> P-8 H-24 O-12
RHS(products) -> P-8 H-24 O-12
4. 2ii) P + O2 => P4O6
(THERE IS A 2 AFTER THE O BECAUSE OXYGEN IS DIATOMIC!!)
surely this is a bit easier than that last one
how many more oxygens + phosphorouses do you need on the LHS to get the same on the RHS?

3) not sure what 'driven off' refers to - water has evaporated or is a product in general?
there are 3 tests im aware of:
a) have you come across CuSO4.5H2O (hydrated copper sulphate) crystals? when in the presence of water, they turn blue. when water is removed, they turn white and are said to be anhydrous copper sulphate crystals.
b) cobalt chloride, which is blue, turns pink in the presence of water.
c)water boils at 100C and freezes at 0C. you could boil/freeze the liquid produced and measure the temp, however this really is only suitable if the water is definitely pure water and there is nothing else dissolved in it, so it may be best you use any of the two above.
5. 4) what exactly is x referring to? i think an equation or a piece of data may be missing here?
5) This is the Gas law equation - pV=nRT
The constant, R, is 8.31Jmol-1K-1
Now you need to convert all the data you are given to SI units.

"a student attempted to calculate relative molecular mass of an unknown volatile liquid. 4.17g of the liquid was gently evaporated in an oven at 18C at a pressure of 101kPa. The gas occupied 2.5Odm^3. Calculate the relative molecular mass of gas"

18C needs to be in K. To do this you add 273 to 18.
101kPa needs to be in Pa. To do this you need to multiply by 1000
2.50dm3 needs to be in m3. To do this, divide by 1000.

put these in the equation, along with the constant and rearrange to find n.

now you need to find the RMM, so from the moles equation, Mr= mass/moles
6. (Original post by gomc)
1i) (desired product/ total mr of product)x1000
1ii) is there meant to be another reaction to look at?
For 1ii) there are two reactions the first is 2Mg +O2= 2MgO and the second is Mg+CuO=MgO+Cu
The question is which method of making magnesium oxide will have the higher atom economy

Note: I've never been taught atom economy so I know nothing about it

Posted from TSR Mobile
7. (Original post by halliethestudent)
For 1ii) there are two reactions the first is 2Mg +O2= 2MgO and the second is Mg+CuO=MgO+Cu
The question is which method of making magnesium oxide will have the higher atom economy

Note: I've never been taught atom economy so I know nothing about it

Posted from TSR Mobile
atom economy is the molar mass of the desired product divided by the sum of the molar masses of all the products, expressed as a percentage.
the equation is (molar mass of the desired product / sum of the molar masses of all products) x 100.
so in the case 1ii
2Mg +O2= 2MgO is an addition reaction (the reactants combine to form a single product) the atom economy for addition reactions is always 100%. (this would be the best choice)
and Mg+CuO=MgO+Cu is not - it is a substitution reaction in which at least two products are formed. since you only want MgO and not the Cu, this is more wasteful.

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