# Prelim Chem Thread (1 Viewer)

#### Snowflek

##### Active Member
is it 0.85 times pbno3 molecular weight which gives the moles?

#### HeroicPandas

##### Heroic!
because we have the c=n/v we have c and v where c is 0.5 and v is 0.065 and if you times them together you get 0.0325
That's finding the number of moles of Al2(SO4)3 and you cannot use this to find the moles of PbSO4 because it's the excess. You must use the limiting reagent to find the moles of PbSO4

• InteGrand

#### HeroicPandas

##### Heroic!
is it 0.85 times pbno3 molecular weight which gives the moles?
Did you mean 0.085?

Ah yeah sorry

#### Snowflek

##### Active Member
We use the pbno3 moles because pbno3 and pbso4 has same mole ratio right? Or what you said just then, you always use the limiting reagent?

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#### HeroicPandas

##### Heroic!
We use the pbno3 moles because pbno3 and pbso4 has same mole ratio right? Or what you said just then, you always use the limiting reagent?
Sorry, I misunderstood what you were saying.

Pb(NO3)2 and PbSO4 coincidently have a mole ratio of 3:3, so 1:1. This does not mean you should always use the moles of a reactant (which supposedly has same mole ratio to the desired product) to find the moles of a product because it may be in excess.

If you use the moles of excess to find desired moles, then you have over-calculated. That's why we use limiting reagent

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#### Snowflek

##### Active Member
0.085 x (207.2 + 32.07 + (16 x 4)) = 25.77795 OMG IS THIS THE ANSWER????

#### HeroicPandas

##### Heroic!
0.085 x (207.2 + 32.07 + (16 x 4)) = 25.77795 OMG IS THIS THE ANSWER????
yeh

#### Snowflek

##### Active Member
Do we not do like 0.085 x ((207.2 x 3) + (32.07 x 3) + (16 x 3 x 4)) right?
Like what i added in was the mole ratio to it

#### HeroicPandas

##### Heroic!
Do we not do like 0.085 x ((207.2 x 3) + (32.07 x 3) + (16 x 3 x 4)) right?
Like what i added in was the mole ratio to it
Yeh, don't do that.

Pb(NO3)2 and PbSO4 were in 3:3 ratio. Doing that puts it in 3:9 ratio

• eyeseeyou

#### Snowflek

##### Active Member
OO!!! i see! thank you for the good explanations!!! If you are free can you solve one more question for me?

#### HeroicPandas

##### Heroic!
OO!!! i see! thank you for the good explanations!!! If you are free can you solve one more question for me?
Sure

• Snowflek

#### Snowflek

##### Active Member
Calcium bromide and sodium sulfate react in solution and a precipitate of calcium sulfate forms. In a certain experiment the mass of calcium sulfate formed in this reaction was calculated to be 0.80g
a) Write a balanced equation for this reaction:
CaBr2 + Na2SO4 ---> CaSO4 + 2NaBr (correct me if im wrong)
b) When the precipitate was collected, dried and weighed, its mass was only 0.55g. The experiment was repeated several times with similar results. Account for the difference between the calculated mass and the mass of the dried calcium sulfate.

#### HeroicPandas

##### Heroic!
Calcium bromide and sodium sulfate react in solution and a precipitate of calcium sulfate forms. In a certain experiment the mass of calcium sulfate formed in this reaction was calculated to be 0.80g
a) Write a balanced equation for this reaction:
CaBr2 + Na2SO4 ---> CaSO4 + 2NaBr (correct me if im wrong)
b) When the precipitate was collected, dried and weighed, its mass was only 0.55g. The experiment was repeated several times with similar results. Account for the difference between the calculated mass and the mass of the dried calcium sulfate.
Sorry, I don't know this one. I forgot nearly all of this but try going through each step of the experiment and see if you can spot any inconsistencies that arise

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#### Snowflek

##### Active Member
Yep ok Thanks for your help from the previous question • HeroicPandas

#### cottonboy99

##### New Member
Hi guys,

Can I please have some help in regards to setting up the following practicals:

a). Separate CuSO4 from Solution A-(which is mostly likely to be water)
b). Electrolysis of Cu from Solution B-(most likely Copper sulfate)

Also can I please have some help in suggestions for writing up the scientific method on one of these. For the separation in a) I guess filtration will be the technique used.

Thank you all.

#### cottonboy99

##### New Member
If someone could respond to my following question ASAP, that would be appreciated Hi guys,

Can I please have some help in regards to setting up the following practicals:

a). Separate CuSO4 (copper sulfate) from water
b). Electrolysis of Cu (copper) from Copper sulfate

Also can I please have some help in suggestions for writing up the scientific method on one of these. For the separation in a) I guess evaporation will be the technique used.

Thank you all.

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#### Asuetsumi

##### New Member
If someone could respond to my following question ASAP, that would be appreciated Hi guys,

Can I please have some help in regards to setting up the following practicals:

a). Separate CuSO4 (copper sulfate) from water
b). Electrolysis of Cu (copper) from Copper sulfate

Also can I please have some help in suggestions for writing up the scientific method on one of these. For the separation in a) I guess evaporation will be the technique used.

Thank you all.
What even is the aim of experiment is this? Is it to conduct gravimetric analysis of the mixture once it has been separated? Is it to simply determine which separation technique is best for each situation? Or is it simply to separate the mixture successfully?

This will shape how you will set up the practical. If its for gravimetric analysis, you would need additional equipment (ie. weight scale, calculator etc.), than if its simply for separation.

For a) you could use evaporation, but you would lose the water and thus won't retrieve all your substances back. Using simply distillation would be more suitable, but is less practical at school.

#### cottonboy99

##### New Member
What even is the aim of experiment is this? Is it to conduct gravimetric analysis of the mixture once it has been separated? Is it to simply determine which separation technique is best for each situation? Or is it simply to separate the mixture successfully?

This will shape how you will set up the practical. If its for gravimetric analysis, you would need additional equipment (ie. weight scale, calculator etc.), than if its simply for separation.

For a) you could use evaporation, but you would lose the water and thus won't retrieve all your substances back. Using simply distillation would be more suitable, but is less practical at school.
The aim for practical a) would be gravimetric analysis with additional equipment (weight scale, calculator etc.) as you mentioned above.

#### eyeseeyou

##### Well-Known Member
Help

In order to compare the reactivity of three different metals, Macy conducted an experiment where in which she placed the metals in different electrolyte solutions and recorded observations that occurred over time.

1. When metal A was placed in a 0.1M nitrate solution of Metal B there was no reaction observed
2. When metal B was placed in a 0.1M nitrate solution of Metal C a precipitate was seen to deposit on the surface of metal A
3. Metal B was then placed in a 0.1M nitrate solution of Metal C. Macy noticed that a precipitate was deposited on the surface of metal B

Arrange the metals in order of decreasing reactivity and explain the reasoning behind your arrangement (4 marks)