Question #9b912

1 Answer
Oct 31, 2015



The idea here is that you have a sample of zinc and a sample of iodine that combine to form, like the problem tells you, a compound of zinc and iodine.

Now, if you assume that all the zinc and all the iodine that took part in the reaction are now part of the compound, you can say that the mass of the resulting compound will be

#" "m_"compound" = m_"zinc" + m_"iodine"#

#" "m_"compound" = "2.50 g" + "9.70 g" = "12.20 g"#

To find the chemical formula of this compound, you need to figure out how many moles of each element you get in that sample.

Basically, the same principle applies - all the moles of zinc and all the moles of iodine are now part of the compound.

So, how many moles of each did you have in the initial samples?

To find that out, use the molar amsses of two elements

#" "2.50color(red)(cancel(color(black)("g zinc"))) * "1 mole zinc"/(65.38color(red)(cancel(color(black)("g zinc")))) = "0.03824 moles zinc"#


#" "9.70color(red)(cancel(color(black)("g iodine"))) * "1 mole zinc"/(126.9color(red)(cancel(color(black)("g iodine")))) = "0.07644 moles iodine"#

To find the mole ratio that exists between the two elements in the compound, divide both values by the smallest one

#" " "For zinc: " (0.03824color(red)(cancel(color(black)("moles"))))/(0.03944color(red)(cancel(color(black)("moles")))) = 1#

#" " "For iodine: " (0.07644color(red)(cancel(color(black)("moles"))))/(0.03824color(red)(cancel(color(black)("moles")))) = 1.999 ~~ 2#

This means that the empirical formula of the compound will be

#" " "Zn"_1"I"_2 implies "ZnI"_2#

As it turns out, this is also the formula unit of the compound.

For ionic compounds, the empirical formula, which tells you what the smallest whole number ratio between the elements that make up that compound is, is also the formula unit.

SInce you can't have a whole number that's smaller than #1# (and different from zero, of course), it follows that #1:2# is the smallest whole number ratio in which the two elements can be combined.

You can double-check the result by using the molar mass of zinc iodide, #"ZnI"_2#, and the number of moles you have.

Since you have

#" ""Zn" + "I"_2 -> "ZnI"_2#

you can say that every mole of zinc will produce one mole of zinc iodide. If this is the case, the you can expect the mass of the resulting compound to be

#" " 0.03824color(red)(cancel(color(black)("moles"))) * ("319.19 g ZnI"_2)/(1color(red)(cancel(color(black)("mole")))) = "12.2 g ZnI"_2#