[Shea]

Hi.  I need help with a few questions like these.  I am utterly lost.  You see, I'm in an online school, and I just can't understand the lessons they provide.  I've tried asking the teachers, but they take days to respond, and I only have until Aug 11 to complete the remaining 10 lessons.

How many moles of ammonia are produced when 2.5 moles of nitrogen are reacted with hydrogen?

How many moles of butane do we need to produce 10 moles of CO2 by reaction with oxygen?

How many moles of P4O10 are produced when we react 8 moles of phosphorous with oxygen?

Although I would like the answers, I would more appreciate it if someone could tell me simply and clearly how to do them myself.

Please help.

[wereworm73]

The key step with these problems is to write the balanced equation of the reaction.

With the ammonia reaction, we know nitrogen and hydrogen are the reactants and ammonia is the product...So the reaction goes something like this...

N₂ + H₂ --> NH₃

But this equation isn't balanced yet because the number of nitrogen & hydrogen atoms don't match on both sides.  But when you make them match, you'll get this as your equation: 

N₂ + 3 H₂ --> 2 NH₃

Looking at the balanced equation, we see that a mole of nitrogen ends up producing 2 moles of ammonia.  So, if you had 2.5 moles of nitrogen, you'd get 5 moles of ammonia.

[jwesterway]

refer to previous post for ammonia question. Same thing applies for the rest of the questions, find the balanced chemical equation and use the ratio of one component to the other to find your answers.

Butane = C4H10
C4H10 + O2 --> CO2 + H2O (water is produced as a byproduct)
Balancing...
2C4H10 + 13O2 --> 8CO2 + 10H2O
so 2 mole butane reacts with 13 mole oxygen gas to produce 8 mole carbon dioxide gas and 10 mol water (steam due to the heat of the reaction).
so to make 10 mole of CO2 you would need to figure out -->
2:8 = ?:10 or 2/8 = ?/10
? = 10*(2/8) = answer

8P + O2 --> P4O10 (white phosphorus)
Balancing...
8P + 10O2 --> ?P4O10
This answer is fairly easy to get, finish the balancing.

I didn't mean for this to tell you the answer but give you a decent shove toward getting it yourself. The due date is looming, good luck!

-Josh

[Shea]

Wow, I really think that helped!  The so-called teachers never explained it that well.  If I have any more questions such as those I'll be sure to come here.  However, I could use some help on this type of problem, and as before, I need to see each step.

How many atoms of hydrogen are contained in 100g of methane (CH4) ?

I've been so nervous that I won't pass chemistry.  If I don't, I can't graduate.  I need this.

[AWK]

Chamge mass of metane to moles of methane, then to moles of hydrogen atoms. Finally multiply them by an Avogadro number

[Shea]

How many moles of hydrogen are produced when 2.1 moles of magnesium are dropped into hydrochloric acid?

This question seems different from the others.  I don't know how to balance this.

[Alberto_Kravina]

This question seems different from the others.  I don't know how to balance this.

Do you mean that  you can't balance the reaction equation

If not: start with the reaction equation

[Neecze]

First - write equation of reaction:

Mg + 2HCl -> MgCl₂ + H₂

According to that equation:
1 mole of magnesium react with hydrochloric acid to give 1 mole oh hydrogen

Hence:
2,1 moles of Mg - give 2,1 moles of H₂

[Shea]

I'm just really bad at this.  How'd you get that equation? Mg + 2HCl -> MgCl2 + H2

[Alberto_Kravina]

I'm just really bad at this.  How'd you get that equation? Mg + 2HCl -> MgCl2 + H2

....because it is so.....that is the answer

Alkaline earth metals react with acids like that, alkali metals, zinc, iron.. as well

[Neecze]

Well... magnesium and hydrochloric acid are substrats. And in the reaction between reactive metal (especially from first and second group of periodic table) and acid - salt and hydrogen are products. That's all.

[Shea]

How many moles of oxygen are required to react with 0.5 moles of iron to turn all of the iron into rust?

3 moles of carbon are burnt in the presence of 4 moles of oxygen. How much CO2 is produced? What are the limiting and excess reagents? What is the maximum amount of CO2 that can be produced?

Hydrogen fluoride is produced by the action of CaF2 on H2SO4. In a certain process 780 g of CaF2 reacted in excess H2SO4 produces 350 g of hydrogen fluoride. Calculate the theoretical yield and the % yield of this process.


These just keep getting harder.  What do I do?  How do I do it?

[wereworm73]

3 moles of carbon are burnt in the presence of 4 moles of oxygen. How much CO2 is produced? What are the limiting and excess reagents? What is the maximum amount of CO2 that can be produced?

Like before, just start with the reaction equation.

C + O₂ ---> CO₂

This is already balanced so nothing else needs to be done to it.


Now from the equation, we see that each mole of carbon will react with one mole of oxygen to form one mole of carbon dioxide.  But the problem says you have 4 moles of oxygen but only 3 moles of carbon available.  Since you don't have enough carbon to react with all of the oxygen, carbon will be the limiting reagent, limiting the amount of the carbon dioxide product to 3 moles.  The oxygen in this case is the excess reagent, since you'll still have a mole of it left after all the carbon is used up. 

[Borek]

These just keep getting harder.  What do I do?  How do I do it?

First of all: as wereworm73 already pointed out - you have to start with reaction equations. Try to write them. Then you may look at my stoichiometric calculations lectures for some further hints.