Question #d611d

1 Answer
Mar 5, 2017

#"0.50518 moles ions"#


Your strategy here will be to

  • use the moalr mass of magnesium chloride to convert the sample from grams to moles
  • use the chemical formula of the salt to figure out the number of moles of ions produced in solution

So, magnesium chloride has a molar mass of #"95.211 g mol"^(-1)#, which means that #1# mole of this salt has a mass of #"95.211 g"#.

You can thus say that your sample of magnesium chloride will contain

#16.033 color(red)(cancel(color(black)("g"))) * "1 mole MgCl"_2/(95.211 color(red)(cancel(color(black)("g")))) = "0.168394 moles MgCl"_2#

Now, notice that every mole of magnesium chloride contains

  • one mole of magnesium cations, #1 xx "Mg"^(2+)#
  • two moles of chloride anions, #2 xx "Cl"^(-)#

This means that when magnesium chloride dissolves in water, every mole of salt will produce

#"1 mole Mg"^(2+) + "2 moles Cl"^(-) = "3 moles ions"#

in solution. That is the case because magnesium chloride is soluble in water, meaning that it dissociates completely in aqueous solution.

Therefore, you can say that your solution will contain

#0.168394 color(red)(cancel(color(black)("moles MgCl"_2))) * "3 moles ions"/(1color(red)(cancel(color(black)("mole MgCl"_2)))) = color(darkgreen)(ul(color(black)("0.50518 moles ions")))#

I'll leave the answer rounded to five sig figs because that's how many significant figures you have for the mass of magnesium chloride.

Notice that the volume of water has four sig figs, but that we didn't use that value in our calculations.