Rolaids is an over the counter remedy for upset stomachs that functions by react

Question

Rolaids is an over the counter remedy for upset stomachs that functions by reacting with stomach acid. The active ingredients are calcium carbonate (550mg)and magnesium hydroxide (110mg). The stomach can expand to hold up to one liter of food, and when empty, will hold between 45 and 75 ml gastric fluid, which is about 0.1M HCL (aq). The reactions that occur are:

Mg(OH)2 (aq) + HCL (aq) ---> H2O (l) + MgCl2 (aq) (reacts "first")

CaCO3 (aq) + HCL (aq) ---> H2O (l) + CaCl2 (aq)       (reacts "second")

If a person took 2 rolaids while their stomach contained 65.mL gastric fluid, find how much HCl (aq) is consumed and how much, if any, of the Rolaids active ingredients (in mg and identify which one) remain.

Explanation / Answer

The reactions that occur are:

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

CaCO3 (aq) + HCl (aq) ---> H2O (l) + CaCl2 (aq)   

Two rolaids:  

Calcium carbonate 2* 550mg =1.1g = 0.011 moles of CaCO3

magnesium hydroxide 2*110mg = 0.22g = 0.0378 moles of Mg(OH)2

Concentration of each compound in 65ml of gastric fluid:

(0.011*1000ml)/65ml = 0.169 M of CaCO3

(0.0378*1000)/65 = 0.581M of Mg(OH)2

With these results, as concentration of HCl = 0.1M we conclude that ALL HCl PRESENT IN GASTRIC FLUID IS CONSUMED BY REACTION WITH Mg(OH)2. Not only that, but we can find that:

Excess of Mg(OH)2=0.581M-0.1M=0.481M

65 mL of a 0.481M solution, are 0.0313 moles. Equivalent to 1.82g =1823mg

Excess of CaCO3 is all of it! Means 550mg x2=1100 mg

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