Posted by John on .
There are two naturally occurring isotopes of chlorine with ratios of 76% and 24%. One mole of PCl3 is made using a sample of isotopically pure chlorine gas (containing only one of the two isotopes). The product weighs 142 g. If chlorine gas containing the other isotope were used, one mole of the PCl3 would weigh?
Answer = 136g

first year chem 
Steve,
Well, since P weighs 31g and the isotopes of chlorine weigh 35 and 37
So, one mole of PCl3 is either
31+3*37=142
or 31+3*35 = 136