What mass of magnesium contains the same number of atoms as 7g of iron?

1 Answer
Jan 15, 2018

#3#g

Explanation:

#n = m/M#

number of moles = mass / molar mass

molar mass of #Fe: 56# g/mol

(this is the same as its atomic mass - the sum of masses of its protons and neutrons.)

mass of #Fe# given: #7#g

#n = 7 #g#/56# g/mol# = 0.125# mol

#7#g of iron contains about #0.125# moles of iron.

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#n = m/M -> m = Mn#
mass = molar mass * number of moles

the same number of moles means that there are the same number of atoms, since a mole of atoms is a fixed number of atoms. (precisely #6.02 * 10^23#)

mass of magnesium = molar mass of magnesium (in g/mol) * #0.125# moles

#m = 0.125 * 24 = 24/8#

#m = 3#g

#3#g of magnesium would contain the same number of moles of atoms, and therefore the same number of atoms, as #7#g of iron.