So I am reviewing for my final on Wednesday, and this question just has me baffled:

 

What is the limiting reactant when 31.3 g of manganese (II) chloride, 48.3 g of chlorine, and 25.7 g of water react to produce manganese (IV) oxide and how much hydrochloric acid is produced?

 

I determined that MnCl2 is the limiting reactant. After balancing my equation, I get:

 

31.3(1/126)(8/1)(36)=70 something

 

The answer it is marking as correct is 36.3g. What am I doing wrong? Thanks.

I can't really check your answer without a balanced equation. I'm not too sure of the reaction here. However, the steps should go like this:

 

1: calculate the mass of HCl produced assuming manganese chloride is LR

2: calculate mass of HCl produced assuming chlorine is LR

3: calculate mass of HCl produced assuming water is LR

4: choose the smallest number

 

each of steps 1-3 can be in turn broken into smaller steps:

 

a) calculate number of moles of reactant by dividing mass by molar mass

b) calculate number of moles of product (HCl) by using a stoichiometric conversion factor

c) calculate mass of HCl by multiplying by molar mass

MnCl2 + 3Cl2 + 4H2O ----> MnO4 + 8HCl

 

I am pretty sure I balanced it correctly.

check the formula for manganese (IV) oxide. Remember the (IV) states the charge on the Mn ion, not the number of oxides.

Ah, it is MnO2. Is that because the molecular formula is Mn4O2, then when you do "cris cross" it is Mn2O4, empirical formula MnO2?

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I rebalanced it and am getting the correct answer. Thanks for pointing that out! I had forgotten that the roman numeral meant the charge and not the amount.

i didn't follow your reasoning about the "crisscross" and the molecular and empirical formula (this is an ionic compound so it doesn't have a molecular formula, because there are no molecules). The only way to reason it out is that the (IV) means the Mn is Mn4+ and you should know oxygen is almost always O2- in ionic substances, so to balance the charges you must have 2 oxides for every manganese.

Ok, your reasoning is the correct one and parallels mine, so I understand. Thanks for your help