How many atoms are there in 7.55 g Si?

It's basically the same thing, we just need to use sodium's molar mass instead of silicon's.

43.7 g / 22.990 g/mol = 1.900826446 mol Na

This time I'll be rounding it again, to 1.901 mol Na. We only need to round at the end, due to significant figures, but it's much easier than typing it all out unless you have a calculator that allows you to input the previous answer such as TI-89.

1.901 mol Na * 6.022x10^23 mol^-1 = 1.1447822x10^24 atoms Na

Because we have three figures in the starting measurement, we also need three figures in the answer. Make sure to round the last digit.

1.14x10^24 atoms Na

I highly suggest memorizing Avogadro's Number for these sorts of calculations, as well as keeping a periodic table with all of the molar masses on hand. Something like WebElements can also be useful.

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:

How many moles of atoms are there in 43.7g Na?