So first, we need to figure out how many moles there are.
7.55 g / 28.085 g/mol = 0.2688267758589995 mol
Rounding that to 0.269 mol for simplicity (it's better not to round it though, so avoid it if you can), let's multiply it by Avogadro's Number.
0.269 mol x 6.022x10^23 mol^-1 = 1.619918x10^23 atoms
Now we need to take significant figures into account. 7.55 has three figures, so the answer must have three figures. Round the third figure as well.
1.62x10^23 atoms
I hope that was useful. c:
