1) First, let's write the chemical equation:
2 NaCl + Pb²⁺ ---> PbCl₂ + 2 Na⁺
2) Let's calculate the number of moles in 10.9 g of PbCl₂. You can obtain using the following equation:
Moles = mass/molar mass
Mass of PbCl₂ = 10.9
Molar mass of PbCl₂ = 207 + (35.5x2) = 278 g/mol
Moles of PbCl₂ = 10.9/278
Moles of PbCl₂ = 0.039 moles
3) How many moles of NaCl are needed to form 0.04 moles of PbCl₂? To find out, we need to use the proportion of the 1) reaction.
2 moles of NaCl ---- 1 mol of PbCl₂
x moles of NaCl ---- 0.039 mol of PbCl₂
1x = 0.078
x = 0.078 moles
Answer: The number of moles of NaCl is 0.078 moles.