The process for determining the number atoms in "0.390 g P"_2"O"0.390 g P2O involves the following steps:
"molar mass"molar massrarr→"moles of molecules"moles of moleculesrarr→"number of molecules"number of moleculesrarr→"number of atoms"number of atoms
Determine molar mass of "P"_2"O"P2O.
Determine the molar mass of "P"_2"O"P2O. I looked it up online and found it to be "77.947 g/mol"77.947 g/mol (rounded to five significant figures).
http://www.webqc.org/molecular-weight-of-P2O.html
Determine moles of molecules "P"_2"O"P2O
In order to determine moles of "P"_2"O"P2O, we divide the given mass in the question by the molar mass.
(0.390color(red)cancel(color(black)("g")))/(77.947color(red)cancel(color(black)("g"))/"mol")="0.0050034 mol P"_2"O"
I am leaving some extra digits in the number of moles to reduce rounding errors. I will round the final answer to three significant figures.
Determine number of molecules.
Multiply the moles of "P"_2"O" by 6.022xx10^23 molecules.
0.0050034color(red)(cancel(color(black)("mol")))xx(6.022xx10^23"molecules")/color(red)(cancel(color(black)("mol")))=3.0130xx10^21"molecules"
Determine number of atoms of "P"_2"O".
Since every molecule of "P"_2"O" consists of three atoms, multiply 3.0130xx20^21"molecules" by "3 atoms/molecule".
3.0130xx10^21 color(red)cancel(color(black)("molecules"))xx(3color(white)(.)"atoms")/color(red)(cancel(color(black)("molecule")))=9.04 xx10^21color(white)(.)"atoms" (rounded to three significant figures)
