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chafri
How many moles are in 3.50 x 1024 atoms of yttrium?
multiply your number which is atoms/1 by 1 mol/ 6.02 x 10^23 atoms. and because when multiplying fractions the numbers and the units get multiplied straight across. THen you have atoms/atoms which proceed to cross out and you are left with your number of: 5.95 x 10^-21 moles
divide amount of atoms you have by avogadros number. \[\frac{ 3.5*10^{24} atoms }{ 6.022*10^{23} \frac{ atoms }{ mole }}\] = 5.812 moles