moles (1 Viewer)

[Sien]

can you just write n=m/mm or do you have to write moles= mass/mol.mass

 

[Crisium]

There's a lot of controversy in writing the first way as I have seen teachers interpret it as Number of Moles = Mass / (Mass x Mass) which basically equate to number of moles being equal to the reciprocal of the mass

Alternatively you can write n = m / M.

There is, however, controversy over this as well since M is (by convention) short-hand for mol L^-1

In year twelve I don't think that they expect you to write out the formula when carrying out the calculation; you're rewarded for the correct substitution, rounding off to the correct number of significant figures, etc.

But in year 11 if you are expected to recite the formula at the beginning of every question then there's no harm in writing the whole thing out

^ If you're one of those people who don't want to waste a few seconds in an exam to do this, then you should ask your teacher and see what he/she prefers/allows

 

[InteGrand]

If you write molar mass as MM (capital m's) and mass as m (lowercase m), they shouldn't be able to interpret it as mass/mass², nor get confused as M representing mol L^-1 (since you used two M's – you could even write M.M. (with dots) to be really pedantic).

Or you could write like n(CO₂), m(X) and MM(X), where X is the species you are referring to (e.g. MM(CO₂) would refer to "molar mass of carbon dioxide" etc.) (this is probably better to use as there may be cases where you need to refer to molar masses etc. of several species for a calculation).

 

[InteGrand]

Also, the marking criteria for Question 25(b) of the 2014 HSC Paper (see page 6 here: http://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-mg-chemistry.pdf ) had ''Correctly uses n × M to calculate mass'' as a criterion.

 

[Librah]

Just do n(X) for moles of x or m(x) for mass of x. Or do a written statement like, "there are x moles of this." You don't have to write down the formula, they're just pronumerals.

 

[someth1ng]

The symbol for molar mass is M, not MM - I think it should be acceptable to mark you down if you use MM.

The correct formula is n=m/M.

 

[snowflakeptical]

can we write n=mass/m.mass?

 

[someth1ng]

can we write n=mass/m.mass?

No.

 

[snowflakeptical]

No.

ok i might as well right the whole thing like: moles, n=mass/molar mass

 

[Crisium]

ok i might as well right the whole thing like: moles, n=mass/molar mass

It shouldn't matter for us 2015'ers because we don't have to recite the formula whenever we do the calculation question; we are awarded for the correct substitutions, rearranging the formula or given values to make the unknown value the subject, solving the equation and finding the unknown value and then rounding off the a certain amount of significant figures (This depends on the lowest amount of significant figures in the question).

^ I would only be worried about reciting the formula when doing stoichiometry in year 11, but if it helps you then go ahead

 

[someth1ng]

ok i might as well right the whole thing like: moles, n=mass/molar mass

As I said before, do n=m/M - nothing else.

 

[snowflakeptical]

It shouldn't matter for us 2015'ers because we don't have to recite the formula whenever we do the calculation question; we are awarded for the correct substitutions, rearranging the formula or given values to make the unknown value the subject, solving the equation and finding the unknown value and then rounding off the a certain amount of significant figures (This depends on the lowest amount of significant figures in the question).

^ I would only be worried about reciting the formula when doing stoichiometry in year 11, but if it helps you then go ahead

don't they like us showing the relationship to how we subbed in the values?

As I said before, do n=m/M - nothing else.

ok

 

[Crisium]

don't they like us showing the relationship to how we subbed in the values?

They can deduce that from where you subbed it in.

For instance lets say that they give you that the number of moles is 5 and the molar mass is 90. Find the mass.

Markers know what the processes are to get to the answer and so lets say that your first line is:

5 = m / 90

They will deduce whether you've subbed in the given values into the correct form.

(Calculations will never be this simple)

 

[snowflakeptical]

They can deduce that from where you subbed it in.

For instance lets say that they give you that the number of moles is 5 and the molar mass is 90. Find the mass.

Markers know what the processes are to get to the answer and so lets say that your first line is:

5 = m / 90

They will deduce whether you've subbed in the given values into the correct form.

(Calculations will never be this simple)

they should or that'll be slack
i miss those calcualtions

 

[D94]

There's a lot of controversy in writing the first way as I have seen teachers interpret it as Number of Moles = Mass / (Mass x Mass) which basically equate to number of moles being equal to the reciprocal of the mass

Alternatively you can write n = m / M.

There is, however, controversy over this as well since M is (by convention) short-hand for mol L^-1

In year twelve I don't think that they expect you to write out the formula when carrying out the calculation; you're rewarded for the correct substitution, rounding off to the correct number of significant figures, etc.

But in year 11 if you are expected to recite the formula at the beginning of every question then there's no harm in writing the whole thing out

^ If you're one of those people who don't want to waste a few seconds in an exam to do this, then you should ask your teacher and see what he/she prefers/allows

There's no 'controversy'. Using M as a shorthand symbol is completely different to using M as a unit (which is what you are saying). In this context, n, m and M are clearly shorthand symbols.

What you are saying is akin to doing something like Distance = Speed x Seconds (which is obviously wrong, and not what we have in this scenario).

Use n=m/M - it's not an 'alternative' - it's what should be used.

 

[Crisium]

There's no 'controversy'. Using M as a shorthand symbol is completely different to using M as a unit (which is what you are saying). In this context, n, m and M are clearly shorthand symbols.

What you are saying is akin to doing something like Distance = Speed x Seconds (which is obviously wrong, and not what we have in this scenario).

Use n=m/M - it's not an 'alternative' - it's what should be used.

There's no controversy for those who have been carrying out stoichiometric calculations for a while but when you are first introduced to it some people struggle to understand the concepts involved and tend to get confused between the two.

Not sure if anyone's realised but OP's in Year 11 and they would have only started carrying out stoichiometric equations recently unless they started the Metal Module earlier on.

I'm saying that he should avoid writing in a form that may confuse him for the sake of simplicity

 

[D94]

There's no controversy for those who have been carrying out stoichiometric calculations for a while but when you are first introduced to it some people struggle to understand the concepts involved and tend to get confused between the two.

Not sure if anyone's realised but OP's in Year 11 and they would have only started carrying out stoichiometric equations recently unless they started the Water Module earlier on.

I'm saying that he should avoid writing in a form that may confuse him for the sake of simplicity

'Controversy' is not the same as 'confusion'. You made it sound like there was a major debate going on in the scientific community.

Go to page 31 of the Chemistry syllabus and it clearly states n=m/M. The molarity is introduced later on page 35 in a different section. So it's unlikely that the OP knows about molarity before knowing how the number of moles should be written, since you need the number of moles to determine molarity.

When the OP learns about molarity, they can then ponder over the difference between a symbol and a unit. One step at a time.

 

[Crisium]

'Controversy' is not the same as 'confusion'. You made it sound like there was a major debate going on in the scientific community.

Go to page 31 of the Chemistry syllabus and it clearly states n=m/M. The molarity is introduced later on page 35 in a different section. So it's unlikely that the OP knows about molarity before knowing how the number of moles should be written, since you need the number of moles to determine molarity.

When the OP learns about molarity, they can then ponder over the difference between a symbol and a unit. One step at a time.

How on earth was I implying that it was a major debate amongst the scientific community?

We're on a fucking community with a majority of its members who are still in school; the only controversy would be amongst these specific people

I know that it's fucking n = m / M I was just warning OP of the different situations that he may face in the calculation questions

To The OP (Because everyone's winging like a little bitch):

* n = m / M

Where:

n = Number of Moles

m = Mass (g)

M = Molar Mass (g)

* When doing calculations involving concentration just keep in mind that they may say that "the concentration of the HCl solution is 0.05 M". The upper-case "M" in this case is representative of the units for concentration, which in its expanded form is mol L^-1.

* The formula involving concentration will be n = cV

Where:

n = Number of Moles

c = Concentration (mol L^-1)

V = Volume (Litres)

* When you do a calculation question to find the concentration do not leave the final units as "M" (e.g. 0.05 M), leave it in its expanded form (e.g. 0.05 mol L^-1)

 

[Librah]

How on earth was I implying that it was a major debate amongst the scientific community?

We're on a fucking community with a majority of its members who are still in school; the only controversy would be amongst these specific people

I know that it's fucking n = m / M I was just warning OP of the different situations that he may face in the calculation questions

To The OP (Because everyone's winging like a little bitch):

* n = m / M

Where:

n = Number of Moles

m = Mass (g)

M = Molar Mass (g)

* When doing calculations involving concentration just keep in mind that they may say that "the concentration of the HCl solution is 0.05 M". The upper-case "M" in this case is representative of the units for concentration, which in its expanded form is mol L^-1.

* The formula involving concentration will be n = cV

Where:

n = Number of Moles

c = Concentration (mol L^-1)

V = Volume (Litres)

* When you do a calculation question to find the concentration do not leave the final units as "M" (e.g. 0.05 M), leave it in its expanded form (e.g. 0.05 mol L^-1)

Relax.

"Controversy is a state of prolonged public dispute or debate, usually concerning a matter of conflicting opinion or point of view." From wikipedia.

 

[someth1ng]

How on earth was I implying that it was a major debate amongst the scientific community?

We're on a fucking community with a majority of its members who are still in school; the only controversy would be amongst these specific people

I know that it's fucking n = m / M I was just warning OP of the different situations that he may face in the calculation questions

To The OP (Because everyone's winging like a little bitch):

* n = m / M

Where:

n = Number of Moles

m = Mass (g)

M = Molar Mass (g)

* When doing calculations involving concentration just keep in mind that they may say that "the concentration of the HCl solution is 0.05 M". The upper-case "M" in this case is representative of the units for concentration, which in its expanded form is mol L^-1.

* The formula involving concentration will be n = cV

Where:

n = Number of Moles

c = Concentration (mol L^-1)

V = Volume (Litres)

* When you do a calculation question to find the concentration do not leave the final units as "M" (e.g. 0.05 M), leave it in its expanded form (e.g. 0.05 mol L^-1)

You should stop.

In your original post, you weren't even outlining any sort of controversy and writing as n=m/M is not an alternative. Your post wasn't succinct and you basically rambled on about how students get confused when all you needed to say was "n=m/M is what should be used".