There is one chloride ion, Cl-, in each formula unit of NaCl. This means there are 5.13 moles of Cl- in 5.13 moles of NaCl. One mole is 6.02 x10^23 particles, so you would calculate the number of chloride ions in 5.13 moles as follows:
5.13 moles x (6.02 x10^23 ions/1 mole) = 3.09 x10^23 ions
When you are trying to solve a problem like this, there are two things that you need to know:
1. How many ions are produced per molecule of the ionic substance
2. 1 mole af anything = 6.02 x10^23
Sometimes there are more than one of an ion per formula unit of the substance. For example, MgCl2 has 2 chloride ions per formula unit so you would need to account for this in the calculation.