Converting grams to moles

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Lead Chromate is obtained as a precipitate 45.6 g in a precipitation reaction.How many moles of PbCrO4 is this?

 

The molar mass of PbCrO4 is 323 g/mol .That is,

1 mol PbCrO4 = 323 g PbCrO4.  (The molar mass can be determined by adding the molar masses of the elements making up the compound:  (Pb  +   Cr  + 4 x O)

Since the mass of 1 mole of PbCrO4 is known, a proportion can be established to find the number of moles of  the PbCrO4 sample:

                                323 g PbCrO4 :  1 mole PbCrO4 ::   45.6 g PbCrO4 :  x moles PbCrO4

or as an equation:

                                       323 g / 1 mole      =    45.6 g  / x moles

tsolving this equation for x yields:

                45.6 g PbCrO4 x 1 mol PbCrO4/323 g PbCrO4 = 0.141 mol PbCrO4

NOTE:  Many experienced chemists will skip setting up the proportion and utilize the final equation that the proportion gives in every case:

               mass of sample  /  molar mass    =  moles of sampe