[(:emily.]

(Original post by bhoy 1888)
wait, is calcium chloride a weak base, i have that in solution it has a pH of 12 regardless of its concN. By mixing this with a solution such as H2 SO4 which has ph nearer 1-2 would surely make the mixture pH less than 0, no...?

Calcium chloride is a salt, and has a pH around 7 as it is the salt of a strong acid and a strong base.

Calcium hydroxide is a strong base and sulphuric acid is a strong acid, so the salt would be neutral.

[ByTheSeaside]

Is anyone else studying the PPA's? I was told they make up 10% of the exam can anyone confirm this? thanks

[deedee123]

(Original post by ByTheSeaside)
Is anyone else studying the PPA's? I was told they make up 10% of the exam can anyone confirm this? thanks

definitely memorise the PPAs, there's always a few questions on them each year.

[ByTheSeaside]

(Original post by deedee123)
definitely memorise the PPAs, there's always a few questions on them each year.

thanks

[rawragee]

(Original post by ByTheSeaside)
thanks

I second this. My teacher told me that there'll definitely be about 6 marks on the PPAs each year and from all the past papers I've done so far I'd agree

[rawragee]

ugh I just did a past paper - and failed! not the best thing to do three days before the exam, it's massively knocked my confidence because I was getting ~70% in all my other ones.
I had a nasty Hep B injection today so I may just blame it on that

[RyJ]

Hey guys, got a couple of questions. Any help would be awesome:

The Avogadro Constant is the same as the number of:
A) molecules in 16 g of oxygen
B) ions in 1 litre of sodium chloride solution; concentration 1mol/l
C) atoms in 24g of carbon
D) molecules in 2 g of hydrogen

I know that the answer is D, but the thing I'm not sure about is what's the difference between atoms and molecules? (This sounds like a really stupid question, but I'm crashing Higher. Might still sound stupid even though I'm crashing it...) I think that - correct me if I'm wrong - molecules is the formula mass (especially when the element is diatomic, hence 2g of hydrogen, because it's H₂). But I'm not sure, if someone could tell me the difference, that'd be great because these questions come up EVERY year...


Which of the following fatty acids is unsaturated?
A) C₁₉H₃₉COOH
B) C₂₁H₄₃COOH
C) C₁₇H₃₁COOH
D) C₁₃H₂₇COOH

Thanks guys!

[(:emily.]

(Original post by RyJ)
Hey guys, got a couple of questions. Any help would be awesome:

The Avogadro Constant is the same as the number of:
A) molecules in 16 g of oxygen
B) ions in 1 litre of sodium chloride solution; concentration 1mol/l
C) atoms in 24g of carbon
D) molecules in 2 g of hydrogen

I know that the answer is D, but the thing I'm not sure about is what's the difference between atoms and molecules? (This sounds like a really stupid question, but I'm crashing Higher. Might still sound stupid even though I'm crashing it...) I think that - correct me if I'm wrong - molecules is the formula mass (especially when the element is diatomic, hence 2g of hydrogen, because it's H₂). But I'm not sure, if someone could tell me the difference, that'd be great because these questions come up EVERY year...


Which of the following fatty acids is unsaturated?
A) C₁₉H₃₉COOH
B) C₂₁H₄₃COOH
C) C₁₇H₃₁COOH
D) C₁₃H₂₇COOH

Thanks guys!

Molecules are groups of atoms, whereas atoms are elements as small as they can get, without breaking them up into neutrons, protons and electrons. eg. if you have an atom which has 6 protons in the nucleus, it's a carbon atom, it cannot be broken up any further without changing it into something different. Molecules are atoms bonded together in some way, eg. H₂ has two hydrogen atoms and H₂O has two hydrogen atoms and one oxygen atom.

For the second one, think of it drawn out. Unsaturated means it contains double bonds. What I do is ignore the -COOH group, take 1 away from the number of carbon atoms and 3 away from the number of hydrogen atoms (because of the CH₃ at the end of the molecule. In A, B and D what you have left follows the general formula C_nH_2n. In C, it is C₁₆H₂₈ so there is less H than there 'should' be, which means there must be a double bond or two to compensate.

Let me know if any of this doesn't make sense!

[TheFOMaster]

(Original post by RyJ)
Hey guys, got a couple of questions. Any help would be awesome:

The Avogadro Constant is the same as the number of:
A) molecules in 16 g of oxygen
B) ions in 1 litre of sodium chloride solution; concentration 1mol/l
C) atoms in 24g of carbon
D) molecules in 2 g of hydrogen

I know that the answer is D, but the thing I'm not sure about is what's the difference between atoms and molecules? (This sounds like a really stupid question, but I'm crashing Higher. Might still sound stupid even though I'm crashing it...) I think that - correct me if I'm wrong - molecules is the formula mass (especially when the element is diatomic, hence 2g of hydrogen, because it's H₂). But I'm not sure, if someone could tell me the difference, that'd be great because these questions come up EVERY year...


Which of the following fatty acids is unsaturated?
A) C₁₉H₃₉COOH
B) C₂₁H₄₃COOH
C) C₁₇H₃₁COOH
D) C₁₃H₂₇COOH

Thanks guys!

A molecule is a substance with more than one atom I guess. Sodium Chloride is a molecule. While Sodium and Chlorine are atoms on there own. All elements exist as atoms EXCEPT Hydrogen,Nitrogen,Oxygen,Flourin e,Chlorine,Bromine and Iodine (maybe Astatine but you don't need to know that. I don't think anyway :P ) as these exist as diatomic molecules. Remember when doing an Avagadro's Constant question with these you have to double the GFM.

For your Second question for Fatty Acid's to work out if there Saturated or Unsaturated I multiply the Carbons by 2 and add 1 which should equal the Hydrogens if it Saturated. If it doesn't it is unsaturated (contains double bonds)

So the answer is: C

Good Luck on Monday

[(:emily.]

(Original post by TheFOMaster)
A molecule is a substance with more than one atom I guess. Sodium Chloride is a molecule. While Sodium and Chlorine are atoms on there own. All elements exist as atoms EXCEPT Hydrogen,Nitrogen,Oxygen,Flourin e,Chlorine,Bromine and Iodine (maybe Astatine but you don't need to know that. I don't think anyway :P )

For your Second question for Fatty Acid's to work out if there Saturated or Unsaturated I multiply the Carbons by 2 and add 1 which should equal the Hydrogens if it Saturated. If it doesn't it is unsaturated (contains double bonds)

So the answer is: C

Good Luck on Monday

Sodium chloride is ionic, so exists in giant lattices

Also for higher you need to know phosphorus exists as P₄ molecules and sulphur exists as S₈ molecules as well as the diatomic ones you listed.

[RyJ]

(Original post by (:emily.)
Molecules are groups of atoms, whereas atoms are elements as small as they can get, without breaking them up into neutrons, protons and electrons. eg. if you have an atom which has 6 protons in the nucleus, it's a carbon atom, it cannot be broken up any further without changing it into something different. Molecules are atoms bonded together in some way, eg. H₂ has two hydrogen atoms and H₂O has two hydrogen atoms and one oxygen atom.

For the second one, think of it drawn out. Unsaturated means it contains double bonds. What I do is ignore the -COOH group, take 1 away from the number of carbon atoms and 3 away from the number of hydrogen atoms (because of the CH₃ at the end of the molecule. In A, B and D what you have left follows the general formula C_nH_2n. In C, it is C₁₆H₂₈ so there is less H than there 'should' be, which means there must be a double bond or two to compensate.

Let me know if any of this doesn't make sense!

So for those types of question (the first one), if it was talking about atoms, and say it was a diatomic element, would you just take the mass as being the quoted formula mass (i.e. for oxygen, 16g)? But for molecules, and it was a diatomic element, you'd take the mass as being - for a diatomic element like oxygen, O₂ - 32g? Or Did I just make this up?

And thanks for explaining the second question too! Makes a lot more sense now!
Thanks for the help Emily

[RyJ]

(Original post by TheFOMaster)
A molecule is a substance with more than one atom I guess. Sodium Chloride is a molecule. While Sodium and Chlorine are atoms on there own. All elements exist as atoms EXCEPT Hydrogen,Nitrogen,Oxygen,Flourin e,Chlorine,Bromine and Iodine (maybe Astatine but you don't need to know that. I don't think anyway :P ) as these exist as diatomic molecules. Remember when doing an Avagadro's Constant question with these you have to double the GFM.

For your Second question for Fatty Acid's to work out if there Saturated or Unsaturated I multiply the Carbons by 2 and add 1 which should equal the Hydrogens if it Saturated. If it doesn't it is unsaturated (contains double bonds)

So the answer is: C

Good Luck on Monday

Thanks!
Brilliant method for working out if the fatty acid is saturated or not by the way!

[(:emily.]

(Original post by RyJ)
So for those types of question (the first one), if it was talking about atoms, and say it was a diatomic element, would you just take the mass as being the quoted formula mass (i.e. for oxygen, 16g)? But for molecules, and it was a diatomic element, you'd take the mass as being - for a diatomic element like oxygen, O₂ - 32g? Or Did I just make this up?

And thanks for explaining the second question too! Makes a lot more sense now!
Thanks for the help Emily

Yeah that's right!

[RyJ]

(Original post by (:emily.)
Yeah that's right!

Ah! Thank God! Honestly, those questions always annoy me in the multiple choice... Shouldn't be a problem now though! Thank you!

[TheFOMaster]

(Original post by (:emily.)
Sodium chloride is ionic, so exists in giant lattices

Also for higher you need to know phosphorus exists as P₄ molecules and sulphur exists as S₈ molecules as well as the diatomic ones you listed.

First compound that came to mind :P

Thanks for that I completely forgot about them

[rawragee]

Hi! (this just popped into my head)
does anyone know how to tell whether a reaction is exothermic or endothermic?

Which of the following represents an
exothermic process?
A Cl2
(g) → 2Cl(g)
B Na(s) → Na(g)
C Na(g) → Na+(g) + e–
D Na + (g) + Cl–(g) → Na+Cl–(s)

it came up in multiple choice in 2008 so I'm worried

Edit: I had a blonde moment and forgot to include the question

[RyJ]

(Original post by rawragee)
Hi! (this just popped into my head)
does anyone know how to tell whether a reaction is exothermic or endothermic?
it came up in multiple choice in 2008 so I'm worried

Exothermic when ∆H is negative (e.g. ∆H = -286 KJ/mol), or if there is any mention of heat being given off.

Endothermic when ∆H is positive (e.g. ∆H = +1300 KJ/mol).

Hope this helped!

[légèreté]

(Original post by rawragee)
Hi! (this just popped into my head)
does anyone know how to tell whether a reaction is exothermic or endothermic?
it came up in multiple choice in 2008 so I'm worried

The enthalpy change (ΔH) for an exothermic reaction is always negative
The enthalpy change (ΔH) for an endothermic reaction is always positive

Essentially, exothermic reactions release energy while endothermic reactions reaction take in energy (usually in the form of heat).

Combustion is probably the easiest example of an exothermic reaction and the ΔH in an enthalpy of combustion calculation will always be a negative value.

[rawragee]

(Original post by RyJ)
Exothermic when ∆H is negative (e.g. ∆H = -286 KJ/mol), or if there is any mention of heat being given off.

Endothermic when ∆H is positive (e.g. ∆H = +1300 KJ/mol).

Hope this helped!

(Original post by légèreté)
The enthalpy change (ΔH) for an exothermic reaction is always negative
The enthalpy change (ΔH) for an endothermic reaction is always positive

Essentially, exothermic reactions release energy while endothermic reactions reaction take in energy (usually in the form of heat).

Combustion is probably the easiest example of an exothermic reaction and the ΔH in an enthalpy of combustion calculation will always be a negative value.

oh no, I didn't include the question! I'll edit my original post aha, but here it is!

Which of the following represents an
exothermic process?
A Cl2(g) → 2Cl(g)
B Na(s) → Na(g)
C Na(g) → Na+(g) + e–
D Na + (g) + Cl–(g) → Na+Cl–(s)

I'm a bit puzzled to be fair

[RyJ]

(Original post by rawragee)
oh no, I didn't include the question! I'll edit my original post aha, but here it is!

Which of the following represents an
exothermic process?
A Cl2(g) → 2Cl(g)
B Na(s) → Na(g)
C Na(g) → Na+(g) + e–
D Na + (g) + Cl–(g) → Na+Cl–(s)

I'm a bit puzzled to be fair

Are you sure it isn't "Which of the following represents an endothermic process"? Because more than one of those is exothermic (unless I'm mistaken...).

If it is an endothermic process they're looking for; then it's D. This is because bonds are created, and when you create bonds, you get energy from the reaction - making it endothermic.

I could be wrong, of course. But in my mind: A, B and C are all exothermic...