Can someone help me find the molar mass of something?

Question

For my science hw, I have to find the molarity of this sunbstance, here I'll show you the problem:

Determine the molarity of the following solution. Show your work to receive credit.
525 g of lead(II) nitrate, Pb(NO3)2, dissolved in 1250 mL of solution.

So heres what I got so far:

Mass = 525 g, Volume = 1250 mL x 1000 = 1250000 L
Moles of Pb(NO3)2 = mass/molar mass
Moles = 525/?

I can't seem to find the molar mass, so does anyone know?

Thanks in advance

Answer 1

Two things;
1250 mL is 1.25L (divide not multiply) ;)
Second, the molar mass is the sum of the atomic weights of each element in a compound with the units of grams per mole.
NO3 = 62 X 2 = 124 + 207.2 (Pb) = 331.2 grams per mole
525 grams / 331.2 grams per mole = 1.585 moles
1.585 moles / 1.25 liters = 1.268M Pb(NO3)2

Answer 2

To find the MOLAR MASS of something you need a PERIODIC TABLE. The periodic table has all the atomic masses (or weights) listed below the chemical symbol.

So, for Lead Nitrate you add the masses of one Pb, two nitrogens, and six oxygens. Depending on what your teacher requires you to use masses up to one, two or three decimal places, the molar mass of Pb(NO)3 is 331.2 g/mole

Molarity is moles per liter.
To get moles of lead nitrate, divide the given 525 by 331.2 and to find the volume in liters, divide the given milliliter in volume by 1000. (note: mL is a smaller measure than Liter, so you divide not multiply)

This should give you molarity M = (525g/331.2g per mole) / (1250 mL/1000mL per L) or a final value of 1.27 M ( please note significant digits)