TBR Gchem Chap 4 Example 4.13

[shffl]

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How many mL of 0.60M HCL are required to neutralize 3g of CaCO3?

A) 50mL
B) 100mL
C) 200mL
D) 300mL

Solution gave an equation:

CaCO3 + 2 HCl --> CaCl2 + CO2 + H2O

I was lost when it got to the mathematical set-up. "Two molecules of HCl are required for every one molecule of CaCO3".

What I didn't understand was in the mathematical set up, why was moles of CO-2/3 multiplied by two?

Thanks

 

[Rabolisk]

Carbonate can accept 2 hydrogens to become carbonic acid (H2CO3) which then decomposes into the more stable CO2 gas and H2O.

 

[BerkReviewTeach]

How many mL of 0.60M HCL are required to neutralize 3g of CaCO3?

A) 50mL
B) 100mL
C) 200mL
D) 300mL

Solution gave an equation:

CaCO3 + 2 HCl --> CaCl2 + CO2 + H2O

I was lost when it got to the mathematical set-up. "Two molecules of HCl are required for every one molecule of CaCO3".

What I didn't understand was in the mathematical set up, why was moles of CO-2/3 multiplied by two?

Thanks

3g CaCO3 when divided by 100g/mole yields 0.03 moles CaCO3. Because you need two moles HCl for every one mole of CaCO3, you need a total of 0.06 moles of HCl to fully neutralize the 0.03 moles of CaCO3.

0.03 moles CaCO3 + 0.06 moles HCl --> 0.03 moles CaCl2 + 0.03 moles CO2 + 0.03 moles H2 O

 

[shffl]

3g CaCO3 when divided by 100g/mole yields 0.03 moles CaCO3. Because you need two moles HCl for every one mole of CaCO3, you need a total of 0.06 moles of HCl to fully neutralize the 0.03 moles of CaCO3.

0.03 moles CaCO3 + 0.06 moles HCl --> 0.03 moles CaCl2 + 0.03 moles CO2 + 0.03 moles H2 O

That's what I thought too. Why then, is the set-up as follow:

2xmoles of CO-2/3 = (M of H+) x (V of H+) =

2x(0.03) = (0.6)x V

V = 0.06/0.6 = 0.1L = 100ml

I must be missing something because I don't understand the bolded part. I'm under the assumption that since there is 2 moles of H+ for every mole of base, then why should the amount of base be multiplied by 2?

 

[Rabolisk]

The question is asking for how much volume of HCl is required. For every mole of CaCO3, you need 2 moles of HCl. You first convert mass of CaCO3 to moles of CaCO3 (mass / molar mass), then you multiply by 2 (2 moles of HCl per 1 mole of CaCO3), then you convert moles into volume (mole HCl / molarity of HCl). You're multiplying by 2 precisely because it takes 2 moles of H+ for every mole of base...