chemistry help molar mass

[kevina]

So jumping into a level chemistry I am self teaching myself and just wanted to double check I'm working stuff out right.

 

 

Questions I have are

 

1. 250 cm3 of a 1.00 mol in a dm-3 solution

 

I did 250/1000=0.25*1 = 0.25

 

Question 2

 

10cm3 of a 2.0 mol dm_3 solution

 

I did 10/1000=0.01*2 (due to mol)= 0.02 mol

 

Question 3 was find the concentrate in mol dm3

 

So 4 mol in 2dm3 of solution I did 4/2=2 mol dm3

 

And 0.0100 mol in 100cm3 of solution. I did 0.0100*10 as it asked cm3 not dm3 giving 0.1mol dm3

 

Finally last two. Again asking concentration 2 mol of NaOH in 4 dm of solution.

 

I worked out molar mass as 80. Then divided by 4 giving 20

 

0.500 mol of HNO3 in 200cm3 of solution

 

Again worked out molar mass as 63/0.2 = 315 then /2 to get 157.5

 

 

I know I got answers right just wanting to see if I working it out the right way or is it coincidence that the numbers match up ?

[hypervalent_iodine]

So jumping into a level chemistry I am self teaching myself and just wanted to double check I'm working stuff out right.

 

 

Questions I have are

 

1. 250 cm3 of a 1.00 mol in a dm-3 solution

 

I did 250/1000=0.25*1 = 0.25

 

Question 2

 

10cm3 of a 2.0 mol dm_3 solution

 

I did 10/1000=0.01*2 (due to mol)= 0.02 mol

 

Question 3 was find the concentrate in mol dm3

 

So 4 mol in 2dm3 of solution I did 4/2=2 mol dm3

 

And 0.0100 mol in 100cm3 of solution. I did 0.0100*10 as it asked cm3 not dm3 giving 0.1mol dm3

 

 

That all seems fine, although the use of cm³, etc., is a confusing choice of units.

 

Finally last two. Again asking concentration 2 mol of NaOH in 4 dm of solution.

 

I worked out molar mass as 80. Then divided by 4 giving 20

 

0.500 mol of HNO3 in 200cm3 of solution

 

Again worked out molar mass as 63/0.2 = 315 then /2 to get 157.5

 

 

I know I got answers right just wanting to see if I working it out the right way or is it coincidence that the numbers match up ?

 

I'm not sure why you need the molar mass in these calculations? If it's asking you for concentration and you're told you have 2 moles in 4 L of solution, the answer is simply 2/4 = 0.5 moles/L. Did you possibly write the question incorrectly here?

[kevina]

I agree the cm3 was a confusing set of units, I'm guessing it was in there to slip me up and teach me to read the question proper.

 

As for the last bit I can clearly see 2/4= 0.5 however I was worried I'd worked it out wrong as the next question 0.5/0.2 which equals 2.5.

 

As I know the answer is 157.5 I had to experiment a bit to get the numbers so I calculated the molar mass divided by 0.2 ( the amount in the solution) then divided by 2 again as there was only 0.5 mold present at the start. The original post was asking if my math was correctly set out or am I missing a super obvious way of working it out and going into it in far to much depth ? You mentioned no need to work out molar mass if you could show me the math that would be great I'm probably missing something really daft. Thanks

[Fuzzwood]

Always look at the units, make them undergo the same operation you are doing on the numbers: mol/L * L = mol. g / g/mol = mol. g/L / g/mol = mol/L

[hypervalent_iodine]

I agree the cm3 was a confusing set of units, I'm guessing it was in there to slip me up and teach me to read the question proper.

 

As for the last bit I can clearly see 2/4= 0.5 however I was worried I'd worked it out wrong as the next question 0.5/0.2 which equals 2.5.

 

As I know the answer is 157.5 I had to experiment a bit to get the numbers so I calculated the molar mass divided by 0.2 ( the amount in the solution) then divided by 2 again as there was only 0.5 mold present at the start. The original post was asking if my math was correctly set out or am I missing a super obvious way of working it out and going into it in far to much depth ? You mentioned no need to work out molar mass if you could show me the math that would be great I'm probably missing something really daft. Thanks

 

The answer for the last one *is* 2.5 moles/L, so I'm not sure where 157.5 is coming from (unless you've given the question incorrectly). As Fuzzwood said, you should always have a look at the units. If you know that you have 0.5 moles in 0.2 L of solution, and your answer needs to be in moles per L, then it's simply a case of dividing the number of moles you have by the given volume, which in this case is 0.5 moles / 0.2 L = 2.5 moles/L.