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#### huskywolf

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##### HCl adjust ph of water problem....
« on: October 08, 2010, 01:06:37 PM »
Hi I have a problem here and I need some tips on how to solve it please....
Ground waters may contain hundreds of mg dm-3CO3-2 ions....

I have a 0.25 L sample of hard water (ground water) ,which is titrated with 0.073 M HCl , the ground water was found to require 0.035L of HCl to adjust the pH to 5.0

I need to calculate the estimated number of moles of CO3-2 in the 0.25L sample of hard water?

So far I have (0.035)(0.073) = 0.0026 M  HCl is used ,I don't know what to do next?

#### DrCMS

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##### Re: HCl adjust ph of water problem....
« Reply #1 on: October 08, 2010, 01:38:13 PM »

How many moles of acid react with each mole of carbonate?

How many moles of acid were used?

So how many moles of carbonate were in the sample?

#### huskywolf

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##### Re: HCl adjust ph of water problem....
« Reply #2 on: October 08, 2010, 02:53:22 PM »
Hello,

I'm not sure which way the equation should go?

2HCl + CO32- = H2CO3 + Cl2+2 ?

For the number of moles of HCl i get seems wrong also:

n=M*v => N(HCl)=(0.0026)(0.035) =0.000091 mol?

#### huskywolf

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##### Re: HCl adjust ph of water problem....
« Reply #3 on: October 08, 2010, 05:17:58 PM »
I got 0.000182 moles of carbonate ?

It looks correct since the the ratio of moles in the equation is 2:1

#### huskywolf

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##### Re: HCl adjust ph of water problem....
« Reply #4 on: October 09, 2010, 04:16:41 AM »
Hi can someone tell me if I am correct please?

I need to be able to do this for tomorrow...

#### DrCMS

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##### Re: HCl adjust ph of water problem....
« Reply #5 on: October 09, 2010, 05:07:21 AM »
So far I have (0.035)(0.073) = 0.0026 M  HCl is used ,I don't know what to do next?

Not quite you've multiplied a molarity by a volume so the answer is moles not moles per litre.

For the number of moles of HCl i get seems wrong also:

n=M*v => N(HCl)=(0.0026)(0.035) =0.000091 mol?

No you have multiplied the no. of moles of HCl calculated above by the volume used.

#### Borek

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##### Re: HCl adjust ph of water problem....
« Reply #6 on: October 09, 2010, 05:17:52 AM »
I'm not sure which way the equation should go?

2HCl + CO32- = H2CO3 + Cl2+2

What does it mean "balanced equation"?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### huskywolf

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##### Re: HCl adjust ph of water problem....
« Reply #7 on: October 09, 2010, 08:20:48 AM »
So far I have (0.035)(0.073) = 0.0026 M  HCl is used ,I don't know what to do next?

Not quite you've multiplied a molarity by a volume so the answer is moles not moles per litre.

For the number of moles of HCl i get seems wrong also:

n=M*v => N(HCl)=(0.0026)(0.035) =0.000091 mol?

No you have multiplied the no. of moles of HCl calculated above by the volume used.

I see, thanks...

Now, I have the number of moles of HCl is 0.0026 , and the number of moles of carbonate is 0.0013 moles...
Is this correct now? Thanks

#### huskywolf

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##### Re: HCl adjust ph of water problem....
« Reply #8 on: October 09, 2010, 08:23:15 AM »
I'm not sure which way the equation should go?

2HCl + CO32- = H2CO3 + Cl2+2

What does it mean "balanced equation"?

The mass on the left must = mass on the right....

Maybe it should look like this? 2HCl + H2CO3=> H2O + CO2+ 2HCl

#### huskywolf

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##### Re: HCl adjust ph of water problem....
« Reply #9 on: October 09, 2010, 09:51:15 AM »
I'm not sure which way the equation should go?

2HCl + CO32- = H2CO3 + Cl2+2

What does it mean "balanced equation"?

The mass on the left must = mass on the right....

Maybe it should look like this? 2HCl + CO3=> H2O + CO2+
2Cl-

Sorry ,, is the number of moles of Carbonate I calculated correct please?

#### Borek

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##### Re: HCl adjust ph of water problem....
« Reply #10 on: October 09, 2010, 10:13:37 AM »
The mass on the left must = mass on the right....

That's only part. You also need identical charge on both sides.

Quote
Maybe it should look like this? 2HCl + H2CO3=> H2O + CO2+ 2HCl

You don't start with H2CO3, also if you have HCl on both sides and in the same amount, you should cancel them.

1.3 mmol looks OK, but IMHO you are far from being able to solve such questions.
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#### huskywolf

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##### Re: HCl adjust ph of water problem....
« Reply #11 on: October 09, 2010, 10:29:57 AM »
The mass on the left must = mass on the right....

That's only part. You also need identical charge on both sides.

Quote
Maybe it should look like this? 2HCl + H2CO3=> H2O + CO2+ 2HCl

You don't start with H2CO3, also if you have HCl on both sides and in the same amount, you should cancel them.

1.3 mmol looks OK, but IMHO you are far from being able to solve such questions.

Thanks for the help,

I just need plenty of practice

#### DrCMS

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##### Re: HCl adjust ph of water problem....
« Reply #12 on: October 09, 2010, 10:40:22 AM »
The balanced equation should be:

2HCl + CO32-  H2CO3 + 2Cl2-

also as  H2CO3  H2O + CO2

For a ground water sample it would most probably be:

2HCl + CaCO3  CaCl2 + H2O + CO2

#### huskywolf

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##### Re: HCl adjust ph of water problem....
« Reply #13 on: October 09, 2010, 11:04:36 AM »
The balanced equation should be:

2HCl + CO32-  H2CO3 + 2Cl2-

also as  H2CO3  H2O + CO2

For a ground water sample it would most probably be:

2HCl + CaCO3  CaCl2 + H2O + CO2

Great, Thank you very much

#### Borek

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##### Re: HCl adjust ph of water problem....
« Reply #14 on: October 09, 2010, 01:52:27 PM »
The balanced equation should be:

2HCl + CO32-   H2CO3 + 2Cl2-