What does Mr stand for in Physics

I have an equation here which states that Mr = NaOH = 40, which would also be equal to the relative molecular mass.

http://en.wikipedia.org/wiki/Molecular_m…

relative molecular mass

thats what you asked but just to add to that ;

its relative because the molecule weighs 40 grams per mol

the molecule (ie just one) weighs a lot less.

Avogadro's number = 6.02214179×10^23 is the number of particles per mol

so 40 / 6.02214179×10^23 = 6.642155133×10^-23 is the actual weight of NaOH

http://en.wikipedia.org/wiki/Mole_%28uni…

relative also being from assuming hydrogen has a mass of 1 atomic unit of mass and then all other atomic masses are with respect to hydrogens atomic mass.

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Mr is Relative atomic mass...It used to be called the molecular weight... you add the atomic weights of the elements together...

Na = 23
O = 16
H = 1

So NaOH = 40

Water H2O would be 1 + 1 + 16 = 18.