Question

Question #60fe9

Answer 1

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