Here's how you'd go about solving these questions.
To get from grams of a substance to moles you need to use molar mass. Molar mass tells what the exact mass of 1 mole of a substance is.
In the case of potassium chloride, its molar mass is listed as 74.55 g/mol. This means that 1 mole of potassium chloride has a mass of 74.55 g.
Since you're dealing with miligrams, mg, you need to convert the mass to grams first, then use the compound's molar mass.
If you want, you can express this value in milimoles
To go from mass to number of molecules, you need to go to moles first, then use Avogadro's number.
According to Avogadro's number, 1 mole of any substance contains exactly
So, to go from molecules to grams, go to moles first, then use ammonia's molar mass.
Once again, use Avogadro's number and the molar mass of gold.
I'll leave parts D and E to you as practice. Don't forget that you get from
- moles to grams and vice versa by using molar mass
- mass/moles to atoms/molecules and vice versa by using Avogadro's number