Help needed for calculations

[Gamewizard]

Hi all,

 

I need some help with these calculations.

 

Q1) 1.2g of urea is dissolved in 500ml of distilled water to prepare a standard. How do i calculate the molar concentration of this standard solution? and I need to express the answer in mmol/l and micromole/l

 

Q2) How can I calculate the urea concentration of a solution after these dilutions below have been made using the standard from Q1.

 

1. 0.1 ml of the standard from Q1 plus 1.9ml water

2. 0.05ml plus 4.95ml water

3. 0.2ml plus 0.8ml water

4. 400microlitre in a total volume of 6ml

 

Q3) What volume of urea stock standard from Q2 is required to prepare standards of the concentrations below?

 

1. 2mmol/l

2. 6mmol/l

3.10mmol/l

4. 0.025mol/l

5. 0.004mol/l

 

Q4) Last question, if I have 2 solutions of NaCl at 200mmol/l and KCl at 20mmol/l, how can I go about preparing mixed solutions to give the following final concentrations of Na and K:-

 

1. 100 mmol/l Na 10mmol/l K

2. 100 mmol/l Na 5 mmol/l K

3. 120 mmol/l Na 4 mmol/l K

4. 45 mmol/l Na 8 mmol/l K

 

Final volume for each needs to be approximately 300ml.

 

Sorry as this is so long, but if someone could show me the calculations involved, not asking for the answers, but calculations and explanation so can work them out myself.

Thank you

[mississippichem]

Show us some of your attempted work. For the first question, try to covert grams of urea to moles of urea. I'll try to address the other questions after you get the first as the rest of the questions depend on that value.

[Gamewizard]

Show us some of your attempted work. For the first question, try to covert grams of urea to moles of urea. I'll try to address the other questions after you get the first as the rest of the questions depend on that value.

 

 

Okay, thanks I will get get back to you tmrrw, but I am not sure how to convert grams of urea to moles of urea ? ?

[hypervalent_iodine]

Okay, thanks I will get get back to you tmrrw, but I am not sure how to convert grams of urea to moles of urea ? ?

 

Do you know how to work out the molar mass of a compound?

[Gamewizard]

Do you know how to work out the molar mass of a compound?

 

Is this the same as Molarity ? then yes (divide moles of solute with litres of solution )

 

OKay, so I attempted this first question. Basically you solve for moles first, take 1.2g of urea and divide by mass of urea which is 60.06.

 

Then solve for litres if given in ml. You divide your ml by 1000

 

Then divide your moles by litres to get the concentration/molarity

 

1.2g/60.06 = 50.05 moles

500ml/ 1000 = 0.5 litres

50.05/0.5 = 100.1 mmol/L Answer

 

Is this correct ? I also need this to be in micromole/L So now I need to figure out how to convert mmol/L into micromole/L

 

Please let me know if this is correct

[CaptainPanic]

Is this the same as Molarity ? then yes (divide moles of solute with litres of solution )

 

OKay, so I attempted this first question. Basically you solve for moles first, take 1.2g of urea and divide by mass of urea which is 60.06.

 

Then solve for litres if given in ml. You divide your ml by 1000

 

Then divide your moles by litres to get the concentration/molarity

 

1.2g/60.06 = 50.05 moles

500ml/ 1000 = 0.5 litres

50.05/0.5 = 100.1 mmol/L Answer

 

Is this correct ? I also need this to be in micromole/L So now I need to figure out how to convert mmol/L into micromole/L

 

Please let me know if this is correct

Almost. The idea is there, but you have some mistakes:

 

You have 1.2 g of urea, and its molar mass is 60.06 g/mol (you must mention the "g/mol" or your answer is wrong... we're not counting eggs, or trees, or meters, or kilograms... we're counting grams/mol. It sounds a bit silly perhaps, but you must write "the (molar) mass is 60.06 g/mol).

 

Then if you calculate 1.2 g / 60.06 g/mol = 0.02 mol (you did 60.06 / 1.2 = 50). You can check this with the units:

 

[math]\frac{g}{\frac{g}{mol}}=mol[/math], so [math]\frac{1.2 g}{\frac{60.06 g}{mol}}=0.02 mol[/math]

 

but:

 

[math]\frac{\frac{g}{mol}}{g}=\frac{1}{mol}[/math]

 

Then you know how many moles of urea you have, and you divide by how many liters (0.5) you have to get "mol/liter" (not yet mmol/liter).

 

p.s. On a sidenote (not related to your question): it took me a long time myself to understand what the hell a "mol" is, and I get a feeling you're still struggling with the idea too. "Mol" is just a number.

I always compare it to a "dozen", which is just 12. You can have 3 dozen eggs (3*12 = 36 eggs). Or you can have 0.5 dozen (half a dozen) eggs, which is just 6 eggs.

You can have 2.5 mol of urea, which is just 2.5*6.022*10^23 = 15.055*10^23 individual molecules of urea... but because that number is so large, we prefer to talk about "mol" only.

Mol is not just for chemistry... you can have 1 mol of elephants, if you want. That just means you have 6.022*10^23 elephants, so you'd better have a really large garden.

[Gamewizard]

Yes thats my biggest weakness in chemistry calculations. Get confused by mole, moles, molarity, units basically etc

 

Ah right so I divided the numbers other way round and got wrong answer.

 

If i do it again, then get 0.5/0.02 mol = 25 mole/L ?

 

 

Now I think I need to know how to convert mole/L to mmol/L and also micromole/L sorry if I am doing something terribly wrong with the units

[hypervalent_iodine]

Yes thats my biggest weakness in chemistry calculations. Get confused by mole, moles, molarity, units basically etc

 

Ah right so I divided the numbers other way round and got wrong answer.

 

If i do it again, then get 0.5/0.02 mol = 25 mole/L ?

 

 

Now I think I need to know how to convert mole/L to mmol/L and also micromole/L sorry if I am doing something terribly wrong with the units

 

If this is for Q1, then no, not quite.

 

You seem to have the calculating your number of moles down, so I'll skip that in the interest of brevity. The best way to figure it out is to look at what units your measurements are in. Molarity is a measure of the number of moles of something you have in a litre of solvent. So for instance, if I have 1 mole of, say, NaCl in 0.5 litres of water then my concentration would be 2M - i.e. 1 mole / 0.5 L. I always used to have trouble remembering which way to divide, but that becomes easy when looking at the units; we know molarity is moles per litre, so therefore to calculate the concentration of a sample we just divide the number of moles by the volume.

[Gamewizard]

If this is for Q1, then no, not quite.

 

You seem to have the calculating your number of moles down, so I'll skip that in the interest of brevity. The best way to figure it out is to look at what units your measurements are in. Molarity is a measure of the number of moles of something you have in a litre of solvent. So for instance, if I have 1 mole of, say, NaCl in 0.5 litres of water then my concentration would be 2M - i.e. 1 mole / 0.5 L. I always used to have trouble remembering which way to divide, but that becomes easy when looking at the units; we know molarity is moles per litre, so therefore to calculate the concentration of a sample we just divide the number of moles by the volume.

 

 

Okay, so the way i understand it, it is 0.02/0.5? that would give me 0.04

[hypervalent_iodine]

And that would be correct!

 

Are you okay with the other questions?

[Gamewizard]

And that would be correct!

 

Are you okay with the other questions?

 

 

Yayyy this just indicates how bad I am at dividing ! as i divide everything the other way round, need to use my brain more.

 

 

 

No I am not really

 

Oh and as the first question's answer is in mol/L, I'll need to convert it to mmmol/l and micromol/l which I am not sure about.

Edited September 27, 2011 by Gamewizard

[CaptainPanic]

Okay, so the way i understand it, it is 0.02/0.5? that would give me 0.04

It becomes easier for yourself if you write the units (a 'unit' is something like "gram", or "gram / mol", or "meter" or "degree Celsius").

 

You want "mol per liter", or "mol / l", so you do 0.02 mol / 0.5 liter to get 0.04 mol / l.

If you do 0.5 liter / 0.02 mol, you get 25 l / mol (and the liter and mol are upside down!).

 

You must write down the units. Always. Terrible things can happen if you don't do this:

Final calculations placed the spacecraft in a trajectory that would have taken the orbiter within 57 kilometers of the surface where the spacecraft likely disintegrated because of atmospheric stresses. The primary cause of this discrepancy was human error. Specifically, the flight system software on the Mars Climate Orbiter was written to calculate thruster performance using the metric unit Newtons (N), while the ground crew was entering course correction and thruster data using the Imperial measure Pound-force (lbf). This error has since been known as the metric mixup and has been carefully avoided in all missions since by NASA. (click here for wikipedia article)

[hypervalent_iodine]

Yayyy this just indicates how bad I am at dividing ! as i divide everything the other way round, need to use my brain more.

 

 

 

No I am not really

 

Oh and as the first question's answer is in mol/L, I'll need to convert it to mmmol/l and micromol/l which I am not sure about.

 

Well you've done the hard part for converting to mmol and micromol. All you have to do work out the conversion factor, which a simple Google search will give you. Simply, for every mole you have 1000 mmoles (hence milli) and for every millimole you have 1000 micromol. As an example, 0.02 moles would equate to 20 mmol (0.02 x 1000) and 20000 micromoles (20 x 1000 or 0.02 x 1000000).

 

The second question you need to ask yourself how many moles of urea you are taking out, remembering that that in every litre you have ~ 0.04 moles, and then work out the new concentration using the same process as in question 1.

 

The third question is kind of the reverse of the above, assuming you have 1 litre of your new solutions (2mmol, etc.). For a.), you know you have 2 mmol of urea in every litre of solution. To make this, how many mL of your stock solution will you need, baring in mind that you have 0.04 moles in every litre of stock solution?

 

The fourth question should come easily after the above two, but if you're stuck feel free to ask for help

[Gamewizard]

It becomes easier for yourself if you write the units (a 'unit' is something like "gram", or "gram / mol", or "meter" or "degree Celsius").

 

You want "mol per liter", or "mol / l", so you do 0.02 mol / 0.5 liter to get 0.04 mol / l.

If you do 0.5 liter / 0.02 mol, you get 25 l / mol (and the liter and mol are upside down!).

 

You must write down the units. Always. Terrible things can happen if you don't do this:

 

 

Ah I see it now, this is what I always do wrong !

 

Thank you

[Gamewizard]

Well you've done the hard part for converting to mmol and micromol. All you have to do work out the conversion factor, which a simple Google search will give you. Simply, for every mole you have 1000 mmoles (hence milli) and for every millimole you have 1000 micromol. As an example, 0.02 moles would equate to 20 mmol (0.02 x 1000) and 20000 micromoles (20 x 1000 or 0.02 x 1000000).

 

The second question you need to ask yourself how many moles of urea you are taking out, remembering that that in every litre you have ~ 0.04 moles, and then work out the new concentration using the same process as in question 1.

 

The third question is kind of the reverse of the above, assuming you have 1 litre of your new solutions (2mmol, etc.). For a.), you know you have 2 mmol of urea in every litre of solution. To make this, how many mL of your stock solution will you need, baring in mind that you have 0.04 moles in every litre of stock solution?

 

The fourth question should come easily after the above two, but if you're stuck feel free to ask for help

 

Thanks I will

[imatfaal]

H/T to MissiChem, HyperI and CaptP for an exemplary homework tutorial - I have green buttoned!