Question #60fe9

1 Answer
May 25, 2016

Answer:

Element #"A"# is carbon.

Explanation:

The idea here is that you can determine the identity of element #"A"# by calculating its molar mass.

To do that, you basically need to know two things

  • how many grams of element #"A"# you have in a given sample of #"A"_2"O"#
  • how many moles of #"A"# you have in the aforementioned sample

So, your compound is said to be #60%# #"A"# and #40%# #"O"# by mass. If you were to pick a #"100-g"# sample of this compound, you could say that it contains

  • # "60 g A"#

  • #"40 g O"#

Use the molar mass of oxygen to determine how many moles you have in this sample

#40 color(red)(cancel(color(black)("g"))) * "1 mole O"/(16color(red)(cancel(color(black)("g")))) = "2.5 moles O"#

Now, take a look at the compound's chemical formula. One mole of #"A"_color(red)(2)"O"# contains

  • #color(red)(2)color(white)(a) "moles of A"#

  • #"1 mole of O"#

So, if this #"100 g"# sample contains #2.5# moles of oxygen, it follows that it must contain

#2.5 color(red)(cancel(color(black)("moles O"))) * (color(red)(2)color(white)(a)"moles A")/(1color(red)(cancel(color(black)("mole O")))) = "5 moles A"#

Since the sample contains #"60 g"# of #"A"#, it follows that one mole of #"A"# will have a mass of

#1 color(red)(cancel(color(black)("mole A"))) * "60 g"/(5color(red)(cancel(color(black)("moles A")))) = "12 g"#

This means that the molar mass of #"A"# is equal to

#"molar mass of A" = "12 g mol"^(-1)#

A quick look in the periodic table will reveal that the unknown element is carbon, #"C"#. Your unknown compound is dicarbon monoxide, #"C"_2"O"#.

https://en.wikipedia.org/wiki/Dicarbon_monoxide