# Moles of solution questionsWatch

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#1
Hello, can you help me with how to work out the following questions please? I'll show the method I used to work them out but I am not sure whether it is right or not:

25cm3 of 1.0 mol dm-3 HCl = 0.025*1=0.025
50cm3 of 0.5 mol dm-3 HCl = 0.05*0.5=0.025
250cm3 of 0.25 mol dm-3 HCl = 0.25*0.25=0.0625
500cm3 of 0.01 mol dm-3 HCl = 0.5*0.01=0.005
25cm3 of 1.0 mol dm-3 NaOH = 0.025*1=0.025

Am I working them out correctly?

Thanks 0
#2
How would you calculate the mass of material in given volume of solution? e.g. 25cm3 of 1 mol dm-3 HCl and 50cm3 of 0.5 mol dm-3 NaCl?

Also, how would you work out the concentration in m,oles of dm-3? e.g. 3.65g of HCl in 1000cm3 of solution; 3.65g of HCl in 100cm3 of solution; 6.62g of Pb(NO3)2 in 250cm3 of solution; 1.00g of NaOH in 250cm3 of solution?
1
7 years ago
#3
(Original post by muffin_selight)
Hello, can you help me with how to work out the following questions please? I'll show the method I used to work them out but I am not sure whether it is right or not:

25cm3 of 1.0 mol dm-3 HCl = 0.025*1=0.025
50cm3 of 0.5 mol dm-3 HCl = 0.05*0.5=0.025
250cm3 of 0.25 mol dm-3 HCl = 0.25*0.25=0.0625
500cm3 of 0.01 mol dm-3 HCl = 0.5*0.01=0.005
25cm3 of 1.0 mol dm-3 NaOH = 0.025*1=0.025

Am I working them out correctly?

Thanks Yeah, all of those are correct. 3.65 g of HCl in 1000 cm^3 of solution. What is the concentration in mol dm^3?
Concentration = Moles / Volume

Moles = 3.65/36.5 = 0.1

Volume = 1000 cm^3 = 1 dm^3

Concentration = 0.1 / 1 = 0.1 mol dm^3
0
#4
(Original post by Tullia)
Yeah, all of those are correct. Concentration = Moles / Volume

Moles = 3.65/36.5 = 0.1

Volume = 1000 cm^3 = 1 dm^3

Concentration = 0.1 / 1 = 0.1 mol dm^3
Why did you divide by 36.5?
0
7 years ago
#5
(Original post by muffin_selight)
Why did you divide by 36.5?

As moles = mass/mr, he divided the mass by the Mr of HCl which is 35.5 (Cl) + 1(H) 0
#6
Thanks, I understand now. How would I do 3.65g of HCl in 100cm3 of solution then? I'm stuck on converting the units.. 0
#7
Also, how do I calculate the mass of material in the given volume of solution? I found the following equations:

molarity = concentration in grams per 1000cm3/Mr of the material dissolved
number of moles of material in a given volume = (molarity x volume(cm3))/1000
mass of material in a given volume of solution = (molarity x volume(cm3) x Mr)/1000

But they are just confusing me further I tried working out a question, I'll show you my working and can you tell where I am going wrong please.

25cm3 of 1mole dm-3 HCL

So I worked out the molarity first...

25cm3 x 4 = 1000cm3
1000cm3 / 36.5 = 27.4

Then I put this in the formula: (molarity x volume x Mr)/1000

(27.4 x 25 x 36.5)/1000 = 25

Can you PLEASE tell me how to do it, I am getting so frustrated because I can't do it, I feel like giving up *sigh* 0
7 years ago
#8
(Original post by muffin_selight)
Thanks, I understand now. How would I do 3.65g of HCl in 100cm3 of solution then? I'm stuck on converting the units.. Concentration = Moles / Volume

Moles = 3.65/36.5 = 0.1 (36.5 being the Mr of HCl)

Volume = 100 cm^3 = 0.1 dm^3 (as you divide by 1000 to get from cm^3 to dm^3)

Concentration = 0.1 / 0.1 = 1 mol dm^3

Also, how do I calculate the mass of material in the given volume of solution? I found the following equations:

molarity = concentration in grams per 1000cm3/Mr of the material dissolved
number of moles of material in a given volume = (molarity x volume(cm3))/1000
mass of material in a given volume of solution = (molarity x volume(cm3) x Mr)/1000

But they are just confusing me further

I tried working out a question, I'll show you my working and can you tell where I am going wrong please.

25cm3 of 1mole dm-3 HCL

So I worked out the molarity first...

25cm3 x 4 = 1000cm3
1000cm3 / 36.5 = 27.4

Then I put this in the formula: (molarity x volume x Mr)/1000

(27.4 x 25 x 36.5)/1000 = 25

Can you PLEASE tell me how to do it, I am getting so frustrated because I can't do it, I feel like giving up *sigh*
OK, so the mass of a solute within a given volume is its density. In terms of equation this is Density = Mass / Volume.

You have the mass of HCl as 36.5 (molar mass). You have the volume in dm^3 of your solution as 25/1000 = 0.025 dm^3.

You merely divide the two sums to get the density. In this case 36.5/0.025 = 1460 g/dm^3 (1.46 g/cm^3).
0
#9
(Original post by Tullia)
Concentration = Moles / Volume

Moles = 3.65/36.5 = 0.1 (36.5 being the Mr of HCl)

Volume = 100 cm^3 = 0.1 dm^3 (as you divide by 1000 to get from cm^3 to dm^3)

Concentration = 0.1 / 0.1 = 1 mol dm^3

OK, so the mass of a solute within a given volume is its density. In terms of equation this is Density = Mass / Volume.

You have the mass of HCl as 36.5 (molar mass). You have the volume in dm^3 of your solution as 25/1000 = 0.025 dm^3.

You merely divide the two sums to get the density. In this case 36.5/0.025 = 1460 g/dm^3 (1.46 g/cm^3).
Thanks, how comes you don't do anything with the moles? e.g. 25cm3 of 1 MOLE dm-3 HCl
0
#10
Hang on, wouldn't you use the mass equation: mass=concentration/volume?

So for 25cm3 of 1 mol dm-3 HCl, wouldn't you do 25x1?
0
#11
Can you check whether these are correct please?

Calculating the mass ofmaterial in the given volume of solution:

50cm3 of 0.5mol dm-3 NaCl
58.5/(50/1000)=1170g/dm3

100cm3 of 0.25mol dm-3 of NH4NO3
80/(100/1000)=800g/dm3

25cm3 of 1 mol dm-3 BaCl2
208.3/(25/1000)=8332g/dm3

Calculating the concentration in moles dm-3 of the following:

6.62g of Pb(NO3)2 in 250cm3 solution
6.62/331.2=0.02
0.02/0.25=0.08mol/dm3

1.00g of NaOH in 250cm3 of solution
1/4-=0.025
0.025/0.25=0.1mol/dm3

1.58g of KMnO4 in 250cm3 of solution
1.58/158=0.01
0.01/0.25=0.04mol/dm3

Is my working correct?
0
7 years ago
#12
(Original post by muffin_selight)
Calculating the mass ofmaterial in the given volume of solution:

50cm3 of 0.5mol dm-3 NaCl
58.5/(50/1000)=1170g/dm3

Incorrect. It is Moles = (Concentration)(Volume)/1000. So (0.5)(50)/1000 = 0.025. Then Moles x Molar Mass = Mass. So (0.025)(58.5) = 1.4625 g

100cm3 of 0.25mol dm-3 of NH4NO3
80/(100/1000)=800g/dm3

Incorrect. See above.

25cm3 of 1 mol dm-3 BaCl2
208.3/(25/1000)=8332g/dm3

Incorrect. See above.

Calculating the concentration in moles dm-3 of the following:

6.62g of Pb(NO3)2 in 250cm3 solution
6.62/331.2=0.02
0.02/0.25=0.08mol/dm3

Correct.

1.00g of NaOH in 250cm3 of solution
1/4-=0.025
0.025/0.25=0.1mol/dm3

Correct.

1.58g of KMnO4 in 250cm3 of solution
1.58/158=0.01
0.01/0.25=0.04mol/dm3

Correct.
Hope that helps. 0
2 years ago
#13
Why don't you divude by 1000 insted of 1.
0
2 years ago
#14
(Original post by Abdullaalqamzi)
Why don't you divude by 1000 insted of 1.

The only person I see dividing anything by 1 was Tullia, who was working out the concentration of something in 1000 cm3 = 1 dm3, and since conc = n / v (in dm3) it made sense to divide by 1, rather than dividing by 1000/1000.
0
1 year ago
#15
How many moles of hydrochloric acid are in 100cm of 2M hydrochloric acid ?
0
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