2 NABr + Ca(OH)2 = CaBr2 + 2 NaOH How many moles of NaBr are required to react with 187.34 g of calcium hydroxide?

Question

Grateful for any help! :)

Answer 1

First you want to start off by obtaining the moles for the calcium hydroxide, Ca(OH)2 given in the problem.

You are not given the grams or moles for the NaBr and this is what you are trying to find so it is simply trying to see if you can use stoichiometry to find the balanced ratio and proportions to see what can be yieled based on the BALANCED equation.

This is not a question to necessarily determine the limiting reactant.

So what you will do is convert the grams of Calcium hydroxide Ca(OH)2 to moles of this substance by using its atomic weight and dimensional analysis:

First calculate the atomic weight of Ca(OH)2

Ca = 40.078 x 1

O = 16 x 2 (remember you have a 2 on the outside of the OH ion so you multiply by 2)

H= 1 x 2 (remember you have a 2 on the outside of the OH ion so you multiply by 2)

atomic weight for Ca(OH)2 = 74.078g/mol

I like to wait until the end to round the numbers for weights because this increases accuracy and precision.

So now use dimensional analysis until all you have are moles:

187.34 g x 1mol/74.078g

you have to write the atomic weight in the inverse which is just switching the units and writing the number in reverse so that you will have moles as your final units, grams will cancel grams top to bottom.

so 187.34/74.078 = 2.528956 moles

Now that you have moles

Look back at your equation is it balanced?:

2NaBr + Ca(OH)2 = CaBr2 + 2NaOH

2 Na on both sides

2 Br on both sides

1 Ca on both sides

2 OH ions on both sides

So this is a balanced equation

Now we can use stoichiometry to follow the ratio to see what can be yielded:

from the balanced equation 2 moles of the NaBr react with 1 mole Ca(OH)2:

So then as we calculated from the 187.34g of Ca(OH)2 that 2.528956 moles

Now set up a ratio of what will be yielded:

2 NaBr ---> 1Ca(OH)2

X NaBr-----> 2.528956 Ca(OH)2

now cross multiply, divide and solve for X and you will find what the NaBr moles are based on this:

X= 2(2.528956) = 5.0579 moles of the NaBr if reporting it to the nearest significant figures based off of the information given in probem, but if just reporting to nearest whole number it would be 5 moles of the NaBr.