How to find molarity of pure water?

Original post by halim123

Hi

I just do not know how to work out the molarity of pure water?

I know it is 55.5 M but do not know how to do it?

Can anybody guide me please?

Regards

definition of molarity = moles /volume

density of water = 1 g/cm³ at 4ºC

How many grams in 1000 ml?

can you see where to go from here?

(edited 13 years ago)

Reply 2

halim123

Original post by charco

am I right to say that cm^3 is the same as ml?
therefore i think there is 1000g in 1000ml?
mmm sorry if i am wrong?

Original post by charco

definition of molarity = moles /volume

density of water = 1 g/cm³ at 4ºC

How many grams in 1000 ml?

can you see where to go from here?

Reply 3

Original post by halim123

am I right to say that cm^3 is the same as ml?
therefore i think there is 1000g in 1000ml?
mmm sorry if i am wrong?

yes one ml is = to 1cm3, so divide 1000 by the molar mass of water :smile:

Reply 4

halim123

Original post by guitarmike456

I think you want to suggest to use this formula

Molarity = mass / molar mass

therefore Molarity = (1000g/L)/ (18g/mol) = 55.5 g/L = 55.5 M

am i right?

Original post by guitarmike456

yes one ml is = to 1cm3, so divide 1000 by the molar mass of water :smile:

Reply 5

Original post by halim123

I think you want to suggest to use this formula

Molarity = mass / molar mass

therefore Molarity = (1000g/L)/ (18g/mol) = 55.5 g/L = 55.5 M

am i right?

Reply 6

halim123

Original post by charco

Thanks alot

Original post by charco

Reply 7

Original post by halim123

I think you want to suggest to use this formula

Molarity = mass / molar mass

therefore Molarity = (1000g/L)/ (18g/mol) = 55.5 g/L = 55.5 M

am i right?

The above is only true if you have 1dm3 of water though, first divide 1000cm3 by the volume of solution then you can use the above formula to calculate the molarity, right?

Reply 8

Original post by guitarmike456

The above is only true if you have 1dm3 of water though, first divide 1000cm3 by the volume of solution then you can use the above formula to calculate the molarity, right?

Molarity is true whatever the volume...

it's measured in mol dm^-3

i.e. moles per litre

Reply 9

Original post by charco

Molarity is true whatever the volume...

it's measured in mol dm^-3

i.e. moles per litre

i know i'm saying molarity doesn't equal mass/molar mass, this just gives the moles
to find molarity it is moles PER litre so you must multiply the answer of the above equation by the given volume/1000cm3

EDIT; to find molarity it is moles PER litre so you must multiply the answer of the above equation by the 1000cm3/given volume

(edited 13 years ago)

Reply 10

Original post by halim123

I think you want to suggest to use this formula

Molarity = mass / molar mass

therefore Molarity = (1000g/L)/ (18g/mol) = 55.5 g/L = 55.5 M

am i right?

55.5g/l does not mean 55.5M that is the density of the liquid
55.5mol/l is equal to 55.5M

Reply 11

Original post by guitarmike456

55.5g/l does not mean 55.5M that is the density of the liquid
55.5mol/l is equal to 55.5M

density is also independent of volume...

Reply 12

Original post by charco

density is also independent of volume...

yeah, i'm aware of that but you cannot calculate the density OR molarity of a solution "regardless of its volume"...

Reply 13

thesalamander123

I'm doing AS OCR Bio, i dont need to know this yet do i?

Reply 14

Original post by guitarmike456

yeah, i'm aware of that but you cannot calculate the density OR molarity of a solution "regardless of its volume"...

nobody is calculating the density...

.. we are using the density to calculate the molarity, which is independent of the volume.

Reply 15

Original post by charco

nobody is calculating the density...

.. we are using the density to calculate the molarity, which is independent of the volume.

yes we are, and i'm objecting to this formula
"Molarity = mass / molar mass"
and are you honestly telling me that formula is correct...

Reply 16

Original post by thesalamander123

I'm doing AS OCR Bio, i dont need to know this yet do i?

nope don't think so, i'm doing A2 OCR biology and we still haven't come across it :smile:

Reply 17

Original post by guitarmike456

yes we are, and i'm objecting to this formula
"Molarity = mass / molar mass"
and are you honestly telling me that formula is correct...

At no point, or at any stage, did I use or suggest that the formula you are quoting is general.

If you look at the second line of the OPs working he says

therefore Molarity = (1000g/L)/ (18g/mol) = 55.5 g/L = 55.5 M

Notice he is using the mass of 1 litre (or density), which is perfectly legitimate...

and will work for any pure liquid.

Reply 18