I wonder how one knows that Mg(OH)2 will react first and if so it all of it will react before CaCO3 begins to react. We will assume that is so. You know that mL x M = 45 x 0.1M = 4.5 millimols of acid will be used. How much Mg(OH)2 do you have? That's 110 x 2 x (1 g/1000 mg) x (1/58.32) = approx 0.00378 mols Mg(OH)2. If all of the Mg(OH)2 is consumed it will use0.00378 x (2 mol HCl/1 mol Mg(OH)2) = 0.00756 mols HCl and you have only 0.0045 so HCl is the limiting reagent and you will use all of it and have some Mg(OH)2 and all of the CaCO3 left over. The problem's wording suggests that only ONE reactants remains; I may have missed something but I think both remain. Can you take it from here?
The equations given are not balanced. They are
Mg(OH)2 + 2HCl ==> MgCl2 + 2H2O
CaCO3 + 2HCl ==> CaCl2 | H2O + CO2
Rolaids is an over the counter remedy for upset stomachs that functions by reacting with stomach acid. The active ingredients are calcium carbonate (550 mg) and magnesium hydroxide (110 mg). The inactive ingredients are dextrose, flavoring, magnesium stearate,
polyethylene glycol, pregelatinized starch, and sucrose. 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 0.1 M 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 45 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.
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