Verified answer

This is a dimensional analysis problem, as I am pretty sure that you know. The general goal of this problem is for you to convert one thing to another. This is how I would solve it.

1) Know what it is that you are converting:

A reading of the problem tells you that you are trying to convert grams of Mg(OH)2 to grams of magnesium.

2) Write down any ratios that you can use to solve the problem:

Dimensional analysis only works because of ratios. Put another way, the only reason we can convert meters to centimeters is because there is a ratio that says that 100 centimeters equals 1 meter, or 100 cm/1 m

We can also make ratios in this problem:

For example, we know that there is 1 mole of magnesium per 1 mole of Mg(OH)2. You can get one mol of magnesium from every mole of Mg(OH)2. It might be helpful to review the concept of moles if you are unfamiliar with this concept. You can do so here: http://en.wikipedia.org/wiki/Mole_%28unit%29

We also know that 1 mol of Mg(OH)2 is equal to the molar mass of Mg(OH)2, which is found by adding up the molar masses of all the atoms that make up Mg(OH)2. Additionally, 1 mol of Mg is equal to the molar mass of Mg.

3) Solve the problem by applying the ratios to dimensional analysis:

If the above explanation appears confusing, it should become clearer when I actually use dimensional analysis to solve this problem in one long step. You should start out with

grams of Mg(OH)2

and end with

grams of Mg

These ratios will lead you the way, as well as the fact that "units cancel" in dimensional analysis.

This will be clearer in my worked out version of the problem below:

http://picasaweb.google.com/lh/photo/fxCzjC-GegcHQ...

First of all you need to find how many moles of Mg(OH)2 there are. The molar mass of Mg(OH)2 according to your numbers should be 24.31 + 2 x 16.00 + 2 x 1.01 = 58.33 g/mol

Therefore, the number of moles you have of Mg(OH)2 = 97.4g/58.33g/mol = 1.67 mol

Now we know that for every Mg(OH)2, we have one Mg, and 2 O's and 2H's

If we have 1.67 moles of Mg(OH)2, we have 1.67 moles of Mg

Since the molar mass of Mg = 24.31g/mol, we can do 1.67 mol * 24.31g/mol and we get 40.60 grams of Mg

First finished the equation Cu +2H2SO4 > CuSO4 + 2H2O + SO2 This tells you that 196g > 64g (22.4L at stp) and consequently seventy 4.5g > (22.4 x seventy 4.5)/196 = 8.514L at stp ( 8,514 x a million}/273 = (V x 2)/3 hundred So V =(8.514x300)/(273x2) = 4.678L