If I dissolve a Rolaids tablet (550 mg CaCO3 and 110 mg Mg(OH)2) in 25 ml of water and then add 25 ml of 0.1M HCl, what will be the final pH?

Question

Am doing an experiment on antacids and need to know pH of Rolaids tablet in water, followed by pH of mixture after HCl is added. 

Answer 1

To find the final pH, one would want to calculate the final [H+] after addition of the HCl.  Let us look at the reactions taking place.

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

(2)  Mg(OH)₂(s) + 2HCl(aq) ==> MgCl₂(aq) + 2H₂O(l)

 

Next, we can calculate the moles of CaCO₃ and the moles of Mg(OH)₂ in the tablet. Also, we can find moles H⁺ in HCl.

moles CaCO₃ = 550 mg x 1g/1000 mg x 1 mole/100 g = 0.0055 moles CaCO₃

moles Mg(OH)₂ = 110 mg x 1g/1000 mg x 1 mole/58.3 g = 0.00189 moles Mg(OH)₂

moles H⁺ in HCl = 0.025 L x 0.1 mole/L = 0.0025 moles H⁺

 

In each reaction above (1 and 2), 2 moles HCl are neutralized for every 1 mole of CaCO₃ or Mg(OH)₂.

Thus, 0.0055 moles CaCO₃ + 0.00189 moles Mg(OH)₂ = 0.00739 moles total

Moles HCl capable of being neutralized = 0.00739 x 2 = 0.01478 moles HCl 

 

NOTE: the moles of HCl capable of being neutralized is GREATER than the moles of HCl added (0.01478 vs. 0.0025) Thus, all of the HCl has been neutralized.  This is not the standard procedure, as the antacid is usually dissolved in EXCESS HCl and then the excess is back titrated with NaOH find the amount of acid neutralized by the tablet.

 

Calculating the actual pH of the resulting solution in your experiment is difficult because of a few concerns. First, CaCO₃ is very insoluble (as is magnesium hydroxide). Second, carbonates react with acid to produce CO₂ which then reacts with water to form carbonic acid, a weak acid.  About the best one could say at this point is that the pH of the final solution should be slightly alkaline.