Berkeley Review Gen Chem Part I question

[missdoctor]

This is an acid/base example problem that I think they made an error in with the explanation..

The question is:

How many milliliters of 0.60 M HCl are required to neutralize 3.0 grams CaCO3?
A. 50 mL
B. 100 mL
C. 200 mL
D. 300 mL

And their solution is:

Solution
According to the balanced equation, two molecules of HCl are required for every one molecule of CaCO3. The balanced equation is shown below:

CaCO3(s) + 2HCl(aq) --> CaCl2(aq) + CO2(g) + H2O(l)

And they said the setup is:

.60M x volume= (2 x 3g/100g)..... and then solving for volume will get you 100mL.

What I don't get is why they multiplied CaCO3's number of moles by 2, when it's HCl that has the 2 equivalents.

Thanks so much for the help!

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[IntelInside]

This is an acid/base example problem that I think they made an error in with the explanation..

The question is:

How many milliliters of 0.60 M HCl are required to neutralize 3.0 grams CaCO3?
A. 50 mL
B. 100 mL
C. 200 mL
D. 300 mL

And their solution is:

Solution
According to the balanced equation, two molecules of HCl are required for every one molecule of CaCO3. The balanced equation is shown below:

CaCO3(s) + 2HCl(aq) --> CaCl2(aq) + CO2(g) + H2O(l)

And they said the setup is:

.60M x volume= (2 x 3g/100g)..... and then solving for volume will get you 100mL.

What I don't get is why they multiplied CaCO3's number of moles by 2, when it's HCl that has the 2 equivalents.

Thanks so much for the help!

CaCO3 --> Ca(OH)2 + H2CO3 --> Ca+2 + 2OH- + CO2 + H2O

Ca(OH)2 + 2HCl --> CaCl2 + 2H2O

.60M x volume = (2 x 3g/100g)

Now do you see why? Because CaOH2 dissociates into 2 OH-

 

[missdoctor]

CaCO3 --> Ca(OH)2 + H2CO3 --> Ca+2 + 2OH- + CO2 + H2O

Ca(OH)2 + 2HCl --> CaCl2 + 2H2O

.60M x volume = (2 x 3g/100g)

Now do you see why? Because CaOH2 dissociates into 2 OH-

Hm I'm not understanding that.
The reaction is

CaCO3(s) + 2HCl(aq) --> CaCl2(aq) + CO2(g) + H2O(l)

Why did you do CaCO3 --> Ca(OH)2 + H2CO3 --> Ca+2 + 2OH- + CO2 + H2O? And how did you get that? Are you adding water to CaCO3 first?

I mean I see how you can get 2OH from the reaction you used.. but I don't get why you would do that reaction when that's not the one we're given? You're saying CaOH2 dissociates into 2 OH-.. but we're not reacting CaOH2 we're reacting CaCO3 with 2HCl. I'm lost!
​

 

[IntelInside]

Hm I'm not understanding that.
The reaction is

CaCO3(s) + 2HCl(aq) --> CaCl2(aq) + CO2(g) + H2O(l)

Why did you do CaCO3 --> Ca(OH)2 + H2CO3 --> Ca+2 + 2OH- + CO2 + H2O? And how did you get that? Are you adding water to CaCO3 first?

I mean I see how you can get 2OH from the reaction you used.. but I don't get why you would do that reaction when that's not the one we're given? You're saying CaOH2 dissociates into 2 OH-.. but we're not reacting CaOH2 we're reacting CaCO3 with 2HCl. I'm lost!
​

Because CaCO3 is essentially a salt and Salts are ionic compounds that dissociate in water. They create either basic or acidic conditions. We can tell by comparing the conjugates of the respective ions. And H2CO3 will dissociate into H2O and CO2. So I want to neutralize the 2OH-.

Where did you get this question?

 

[missdoctor]

Because CaCO3 is essentially a salt and Salts are ionic compounds that dissociate in water. They create either basic or acidic conditions. We can tell by comparing the conjugates of the respective ions. And H2CO3 will dissociate into H2O and CO2. So I want to neutralize the 2OH-.

Where did you get this question?

I got this from TBR Gen Chem Part I 2009 edition, page 252 , Example 4.13

Hm I guess I don't understand how from the initital reaction you know to double the CaCO3 moles because when it gets mixed with water, it will form 2OHs. Because it's reacting with HCl, not water right?.. or is this something that should be memorized? That certain compounds will always form two equivalents of something so you should always double their moles in calculations?

Thanks by the way for taking the time to explain this to me!