Question

What mass of `BF_3` contains the same number of molecules as a `0.53*g` mass of water?

Answer 1

And so you want the MASS of `BF_3` that corresponds to the same number of molecules in a `0.53*g` mass of water?

Explanation:

`"Moles of water"-=(0.53*g)/(18.01*g*mol^-1)=0.0294*mol`...

Now the mole is simply a number, i.e. `1*mol` specifies `6.022xx10^23` individual molecules of whatever stuff you got.

ANd so we multiply this molar quantity by the molar mass of `BF_3`....

`-= 0.0294*molxx67.82*g*mol^-1=2.0*g`.........