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.

 

[tomcue]

?

Yes, your calculations are correct. In part a), you correctly calculated the mass of the substance when 2x106 atoms of each kind are present. In part b), you correctly calculated the number of atoms in 1kg of the substance by dividing the given mass by the mass of each atom. Your answer shows that there are a very large number of atoms in 1kg of the substance, which is expected since 1kg is a significant amount of matter.