EllJune
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*When 8.1g of sodium chromate is dissolved in 250cm3 of water, what is the concentration of the solution?*

This is my previous attempt -

250cm3/1000 = 0.25 L
8.1g/0.25L = 32.4 g/L

8.1g = 0.0081 L

Answer - 8.1g/(0.25+0.0081) = 32.4081 g/L

My tutor has said I need to use the number of moles (0.05) of sodium chromate and include it in the formula - concentration = moles/volume (dm3). How do I do this?! I can't understand where to even start.

Thank you in advance!
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lqwerty98
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moles of sodium chromate = mass (8.1g) / Mr

Concentration = number of moles / volume in dm3
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EllJune
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(Original post by lqwerty98)
moles of sodium chromate = mass (8.1g) / Mr

Concentration = number of moles / volume in dm3
I must seem super thick right now... in the question, is the 250cm3 the volume?
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Raizelcadres
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(Original post by EllJune)
I must seem super thick right now... in the question, is the 250cm3 the volume?
Lool don't worry.

You have your mass of Na2CrO4 (sodium chromate) = 8.1g

volume given = 250cm3
convert it from cm3 to dm3

Equation to find concentration=n/v(in dm3)

But you need to calculate moles so use the equation n=m/mr

Calculate mr of sodium chromate and put it into n=m/mr
after calculating the number of moles put it into the concentration equation and you have your answer.
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EllJune
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(Original post by Raizelcadres)
Lool don't worry.

You have your mass of Na2CrO4 (sodium chromate) = 8.1g

volume given = 250cm3
convert it from cm3 to dm3

Equation to find concentration=n/v(in dm3)

But you need to calculate moles so use the equation n=m/mr

Calculate mr of sodium chromate and put it into n=m/mr
after calculating the number of moles put it into the concentration equation and you have your answer.
This seem simple enough to follow... once I know what the n is
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EllJune
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(Original post by Raizelcadres)
Lool don't worry.

You have your mass of Na2CrO4 (sodium chromate) = 8.1g

volume given = 250cm3
convert it from cm3 to dm3

Equation to find concentration=n/v(in dm3)

But you need to calculate moles so use the equation n=m/mr

Calculate mr of sodium chromate and put it into n=m/mr
after calculating the number of moles put it into the concentration equation and you have your answer.
I tried to follow your method, my answer looks completely wrong...

moles = 0.05
molar mass = 161.97 g/mol

0.05/161.97 = 0.0003087
0.0003087/0.25 = 0.0012348 dm3

Surely that isn't the answer?
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EllJune
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(Original post by lqwerty98)
moles of sodium chromate = mass (8.1g) / Mr

Concentration = number of moles / volume in dm3

My answer following this is 0.2 dm3... I'm getting completely different answers for every different method!
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lqwerty98
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(Original post by EllJune)
I must seem super thick right now... in the question, is the 250cm3 the volume?
yes, but you will need to divide 250 by 1000 to convert into dm3
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Laila_00
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Alright, first you need to find the molar mass of sodium chromate in order to find the mole.
So (23 x 2) + 52 + (16x4) = 162 <-- these are just the molar mass of each individual element added up together
Next to find the mole number, you use the formula mole = actual mass / molar mass. That is 8.1/162 = 0.05 mol
Now that you have the mole, to find the concentration you need to convert the volume from cm3 to dm3 and since 1 dm3 = 1000 cm3, you will need to divide 250 cm3 by 1000 to give you 0.25 dm3.
Now all you need to do is just use the formula concentration = moles/ volume = 0.05/0.25 = 0.2 mol/dm3 and that's the answer.
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Raizelcadres
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(Original post by EllJune)
I tried to follow your method, my answer looks completely wrong...

moles = 0.05
molar mass = 161.97 g/mol

0.05/161.97 = 0.0003087
0.0003087/0.25 = 0.0012348 dm3

Surely that isn't the answer?
I got the answer as 0.2 moles.
I will try uploading a picture.
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Raizelcadres
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(Original post by EllJune)
I tried to follow your method, my answer looks completely wrong...

moles = 0.05
molar mass = 161.97 g/mol

0.05/161.97 = 0.0003087
0.0003087/0.25 = 0.0012348 dm3

Surely that isn't the answer?
Mass of sodium chromate = 8.1g
Volume= 250/1000
=0.25dm3

Now find number of moles=8.1/162
=0.05

Now to find concentration: C=0.05/0.25
=0.2moles
your answer is 0.2 moles
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111davey1
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Use n=c*v/1000 if v is in cm3

Use n=c*v if v given in dm3

just rearrange for c
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EllJune
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(Original post by Laila_00)
Alright, first you need to find the molar mass of sodium chromate in order to find the mole.
So (23 x 2) + 52 + (16x4) = 162 <-- these are just the molar mass of each individual element added up together
Next to find the mole number, you use the formula mole = actual mass / molar mass. That is 8.1/162 = 0.05 mol
Now that you have the mole, to find the concentration you need to convert the volume from cm3 to dm3 and since 1 dm3 = 1000 cm3, you will need to divide 250 cm3 by 1000 to give you 0.25 dm3.
Now all you need to do is just use the formula concentration = moles/ volume = 0.05/0.25 = 0.2 mol/dm3 and that's the answer.
Thank you so much! I definitely over complicate things
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Kellan123
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I need to convert 2.24moldm-3 to gdm-3 but I am unsure on how to do this?
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