9.76g of a metal form 20.9g of its oxide whose formula is M2O. calculate the relative atomic mass of the metal?

1 Answer
Jan 25, 2018

The molar mass of M is 7.01 g, so it must be lithium.

Explanation:

Here' the way I think this through:

The equation must look like this:

#4M + O_2 rarr 2M_2O#

This is important because now we know that the number of moles of metal is twice as great as the number of moles of the oxide.

Let's let the molar mass of M be #x#, then

moles of M = #9.76 / x# and moles of the oxide = #20.9 / (2x+16)#

Since there is twice as many moles of the metal as the oxide

#9.76/x = 2xx(20.9/(2x+16))#

which can be written #9.76/x = 41.8/(2x+16)#

Solve this by cross-multiplying:

#(9.76)(2x+16)=41.8x#

#19.52x + 156.16 = 41.8x#

#41.8x - 19.52x = 156.16#

#22.28x = 156.16#

#x=7.01#

This is lithium.