Protocol Online logo
Top : Forum Archives: : General Biology Discussion

Concentration help please! - (Aug/10/2007 )

Hi there,

I am really confused when using different notations for concentration...

for example in my book it says..



now what I'm confused about is---are moles and M the same thing? and if so why? is M just short form for mol?

then it gives us a few eg's to do ourself and I dont even get how to do the first one sad.gif---can someone please explain?

--assuming the sample had a conc. of 1.45mM (P04^-3) what would this result be in

ug P/mL (MrP=31)....

am i suppose to use those equations like c=nv/m=nM, if so do i go 0.00145uM*31=0.04495ug/mL--can someone please see if this is right and if not please help me understand how to do such problems

Thanks in advance,


lets start with the basics of chemistry. (Grabed shamlessly from wikipedia-- which may one day be the reporsitory of all human knowledge)

The mole (symbol: mol) is the SI base unit that measures an amount of substance. One mole contains Avogadro's number (approximately 6.022×10^23) entities. A mole is much like "a dozen" in that both are absolute numbers (having no units) and can describe any type of elementary object

Molarity (M) denotes the number of moles of a given substance per litre of solution. @ the number of moles present in a give volume.

So in mol is not M.

mol is the short for mole. (amount)
M is the short for molarity. (concentration)

So imagine molarity (M) this way. If 5 jugs of water have 10 balls in them. How many balls do you have in 1 jug? The answer is 2 balls. So it isn't strange that for a give concentration, a smaller volume will have less stuff, while a larger volume will have more stuff.

1M = 1mol/L = 0.1mol / 0.1L = 1mmol / 1mL = 1pmol/1pL

less subtance in a smaller volume. but keep the ratio of subtance to volume the same and you keep the same concentration (molarity)

M = mol / L

mol = g / MW

L = litre
M = molarity
mol = mole
g = gram
MW = molecular weight

mol = g / MW
mol * MW = g
Thus the number of grams is the number of moles * molecular weight.

Desire = g / L
which is grams per litter

what you have is
M = mol /L

Ignore the(per Litre) for a second. You have mol, how do you change that to grams? Multiple mol with MW. (remember g = mol * MW)


M * MW = (mol / L ) *MW
= (mol *MW ) / L
= g / L

Hence to convert molarity (M) to g / L, you need to multiple the molarity by molecular weight.
Once you have g / L it is merely some work to convert the units. Always work with Scientific notation; 1x 10^-3, 5x10^4
This whole pmol, mmol... the engineering notation only serves to confuse. Do fall into the obvious trap. Convert everything into scientific notation first to work with the numbers. And once you have your result convert it back to enginnering notation.