How Do You Calculate the Mass and Number of Atoms in a Substance?

[JakePearson]

questions
3.
A substance consists of equal numbers of tow kinds of atoms, with masses m1 and m2 respectively.
a) what is the mass of the substance is 2x10⁶ atoms of each kind are present, where m1 = 3.4x10^-26kg and m2 = 1.6x10^-26kg?
answer;
3.4x10^-26kg + 1.6x10^-26kg = 5x10^-26 x 2x10⁶ = 1x10^-19kg

b) how many atoms are there altogether (counting both kinds) in 1kg of the substance?
answer;
1kg / 5x10^-26kg = 2x10²⁵atoms

is this correct

 

[Redbelly98]

(a) looks good.

For (b), note that 5x10^-26kg is the mass of a pair of atoms. So there are 2x10²⁵ pairs of atoms.