need help with mole questions and how to solve them

Question

Determine the mass in grams of 4.65 × 10²¹ atoms of arsenic. (The mass of one mole of arsenic is 74.92 g.)

Answer 1

The key to mole questions and related questions is knowing how to set up your equations. If you can set up equations so that units cancel, you're golden!

Remember: there are 6.02 x 10²³ atoms in one mole

We start with 4.65 x 10²¹ arsenic atoms and we want to get to mass in g. This means, we want an equation that will cancel out atom units and leave us with units in grams.

To do this, we'll use the above relationship in italics (6.02 x 10²³ atoms in one mole) and the molar mass.

4.65 x 10²¹ arsenic atoms x 1 mol arsenic x 74.92 g arsenic = 0.579 g arsenic

6.02 x 10²³ arsenic atoms 1 mol arsenic

I know the fractions are makeshift, but I think they're helpful to visually see how the units cancel. "Arsenic atoms" appear in both the numerator and denominator and so does "mol arsenic" so, both of those units cancel, leaving us with an answer in g arsenic.