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

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To calculate the mass of a substance with equal numbers of two types of atoms, the total mass is determined by summing the individual masses and multiplying by the number of atom pairs. For 2x10^6 atoms of each kind, the mass is calculated as 5x10^-26 kg per pair, resulting in a total mass of 1x10^-19 kg. To find the total number of atoms in 1 kg of the substance, divide 1 kg by the mass of a pair of atoms, yielding 2x10^25 atoms. It’s important to recognize that this total represents pairs of atoms, not individual atoms. The calculations provided are confirmed to be correct.
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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
 
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(a) looks good.

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

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