Can anybody provide some General Chemistry help?

[DrWanahbe]

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I am two weeks into my first prereq, which is general chemistry. I haven't had this stuff in YEARS!!! I'm hoping someone out there can help me, since it's a holiday, and I can't access my TAs or Professor!!

I'm preping my lab for Thursday. We're given an unbalanced equation, which I was able to balance after about 3 solid hours of work. I now need to determine how much of each substance I will need in the lab to make the reaction occur. I have figured out the molar mass of each compound involved, but I know the amounts are way more than the lab is going to let me use, which means I have to get it down to managable ratios. So, here's what I have:

Calcium Nitrate: 164 g/mol
Diammonium Hydrogen Phosphate: 132 g/mol
aqueous nitrate: 17 g/mol
water: 18 g/mol

The reaction will produce hydroxyapatite with a molar mass of 1004 g/mol and Ammonium Nitrate at 80 g/mol.

Any ideas? What do I do next?

Thanks!

 

[QofQuimica]

I am two weeks into my first prereq, which is general chemistry. I haven't had this stuff in YEARS!!! I'm hoping someone out there can help me, since it's a holiday, and I can't access my TAs or Professor!!

I'm preping my lab for Thursday. We're given an unbalanced equation, which I was able to balance after about 3 solid hours of work. I now need to determine how much of each substance I will need in the lab to make the reaction occur. I have figured out the molar mass of each compound involved, but I know the amounts are way more than the lab is going to let me use, which means I have to get it down to managable ratios. So, here's what I have:

Calcium Nitrate: 164 g/mol
Diammonium Hydrogen Phosphate: 132 g/mol
aqueous nitrate: 17 g/mol
water: 18 g/mol

The reaction will produce hydroxyapatite with a molar mass of 1004 g/mol and Ammonium Nitrate at 80 g/mol.

Any ideas? What do I do next?

Thanks!

How much does your lab book tell you to start with? I can't believe that the procedure doesn't give you any amounts. 😕 If they don't, pick an arbitrary amount of each thing. Let's say you have one gram of each reagent.

In any reaction, you generally have all of the reagents but one present in excess and one that gets used up completely. That is called the limiting reagent. So always start by identifying the limiting reagent. You figure out what the limiting reagent is by calculating the number of moles of each reagent, and comparing them. (Take the number of grams of each, which we arbitrarily made to be one gram in this case, and divide that by the MW.) Then compare how many moles of each you have. The reagent with the fewest moles is the LR. But be careful, because if the reagents are not used in a 1:1 molar ratio (look at your balanced equation to determine whether they are), you will have to account for that. (So for example, if you have 3 moles of A and 2 moles of B, you can't conclude that B is the limiting reagent if you consume 2 moles of A for every one mole of B. In that case, A would be limiting, because 3 moles of A would only consume 1.5 moles of B.)

Once you know what the limiting reagent is, convert from moles of LR to moles of the desired product. Do this by multiplying by the mole ratio, which you get from the coefficients in the balanced equation. What goes on top? Well, you'd better put the moles of product on top, or else you're going to wind up with moles^2 of reactant over moles of product, which doesn't make sense. Lastly, you will probably be asked to convert the moles of product into grams. Do this by multiplying by the MW of the product. This is the theoretical yield. However, you will lose product due to incomplete reaction, transfers, spilling, etc., and so your actual yield will be less than the theoretical yield.

Stoichiometry problems like this are a lot easier if you write out each step with units and cancel them out as you go. That will help you avoid making careless errors. Hope this helps, and good luck w/ your lab. 🙂

 

[Sporky]

I am two weeks into my first prereq, which is general chemistry. I haven't had this stuff in YEARS!!! I'm hoping someone out there can help me, since it's a holiday, and I can't access my TAs or Professor!!

I'm preping my lab for Thursday. We're given an unbalanced equation, which I was able to balance after about 3 solid hours of work. I now need to determine how much of each substance I will need in the lab to make the reaction occur. I have figured out the molar mass of each compound involved, but I know the amounts are way more than the lab is going to let me use, which means I have to get it down to managable ratios. So, here's what I have:

Calcium Nitrate: 164 g/mol
Diammonium Hydrogen Phosphate: 132 g/mol
aqueous nitrate: 17 g/mol
water: 18 g/mol

The reaction will produce hydroxyapatite with a molar mass of 1004 g/mol and Ammonium Nitrate at 80 g/mol.

Any ideas? What do I do next?

Thanks!

Very good advice given above - she's right on track.

FYI, some things which may help you - especially if you are a visual person like I am:

www.chembuddy.com - This software helps you balance equations and won't make you take 4 hours to do it!

www.acdlabs.com - Download their freeware called "chemsketch". It allows you to draw reactions and molecules and automatically makes them 3D and names them for you. Great when you get to Organic.

Best wishes!!

Sporky.

 

[DrWanahbe]

Thanks Q and Sporky! Sporky - thanks for the links, I've bookmarked them!

I'm sorry it's taken me so long to get back to this thread! I did get the right equation, just wasn't sure what to do next, but everything worked out in the end.

I'm now preparing for my next test, which HAD BETTER go in the direction I want it too! Two days yet to study...why does Chemistry have to be so challenging? I sure hope it gets better once the basics are grasped!

And the worst part is...I KNOW I can learn this stuff, I just wish I'd hurry up and do it already...