really hard moles question

17

this is the full question but i really need help with the second part
A student is preparing 250cm3 of a standard solution of sodium hydrogencarbonate (NaHCO3)
the student fixes the mistakes in her method and repeats the procedure.
she makes a standard solution with a concentration of 0.30mol dm-3
8.2) calculate the mass of sodium hydrogencarbonate that would be required to make a 250cm3 with a concentration of 0.30 mol dm-3
here is correct calculation for this question
concentration x volume
0.30 x 0.25=0.075moles
molesxmr=mass
0.075 x 84=6.3
8.3
this is the question i do not get
The student pours 100cm3 of the solution into a beaker containing 150cm3 of distilled water.
calculate the conentration of the solution in the beaker
here is the correct calculation
Moles of NaHCO3=0.075 x (100/250)=0.030 mol
Concentration of NaHCo3=0.030/(250/1000)=0.12 mol dm-3
So i am confused here why is the moles 0.0750x(100/250)

Reply 1

johnnyliu8

13

Original post by we knnow

this is the full question but i really need help with the second part
A student is preparing 250cm3 of a standard solution of sodium hydrogencarbonate (NaHCO3)
the student fixes the mistakes in her method and repeats the procedure.
she makes a standard solution with a concentration of 0.30mol dm-3
8.2) calculate the mass of sodium hydrogencarbonate that would be required to make a 250cm3 with a concentration of 0.30 mol dm-3
here is correct calculation for this question
concentration x volume
0.30 x 0.25=0.075moles
molesxmr=mass
0.075 x 84=6.3
8.3
this is the question i do not get
The student pours 100cm3 of the solution into a beaker containing 150cm3 of distilled water.
calculate the conentration of the solution in the beaker
here is the correct calculation
Moles of NaHCO3=0.075 x (100/250)=0.030 mol
Concentration of NaHCo3=0.030/(250/1000)=0.12 mol dm-3
So i am confused here why is the moles 0.0750x(100/250)

from my understanding, the student diluted it so not all 0.075 moles of NaHCO3 are going to be in the diluted 250cm3 solution. therefore in the diluted solution only 100cm3 /250cm3 is NaHCO3. so to find the number of moles in the 100cm3 we times 0.075 (the amount of mole in 250cm3 of NaHCO3) by 100/250 and you know the rest.

Reply 2

we knnow

OP

17

Original post by johnnyliu8

from my understanding, the student diluted it so not all 0.075 moles of NaHCO3 are going to be in the diluted 250cm3 solution. therefore in the diluted solution only 100cm3 /250cm3 is NaHCO3. so to find the number of moles in the 100cm3 we times 0.075 (the amount of mole in 250cm3 of NaHCO3) by 100/250 and you know the rest.

is the 250cm3 from the standard solution also why is it not 250/1000 in stead of 100 to convert cm to dm

Reply 3

johnnyliu8

13

Original post by we knnow

is the 250cm3 from the standard solution also why is it not 250/1000 in stead of 100 to convert cm to dm

huh? No. since there are 0.075 moles of NaHCO3 in 250cm3, how many moles will be in 100cm3?
basically we are trying to find the amount of moles of NaHCO3 in solution that only contains 100/250 of the original solution.

Reply 4

DanielaDilu

1

Original post by we knnow

is the 250cm3 from the standard solution also why is it not 250/1000 in stead of 100 to convert cm to dm

i have a question regarding this same question, where does the 150cm^3 come in? i don't understand why we didn't use that as the volume when doing concentration=mol/vol. Why did we use the 100cm^3?

Reply 5

Methene

16

You begin with a solution of 250cm^3 , containing 0.075 mols. If you take 100cm^3 of that solution, it will contain 0.075 x 100/250 = 0.030 mols. If you add 150cm^3 of water to this, you now have a 250cm^3 solution, but still containing 0.030 mols.

1dm^3 = 1000cm^3

So you have 0.030 mols in 250cm^3 = 0.25dm^3. So you have 0.12 mols in 1 dm^3. Conc. is therefore 0.12 mol dm^-3.

really hard moles question

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