Question

What mass of which reagent is left unreacted?Hydrogen peroxide is used as a cleaning agent in the treatment of cuts and abrasions for several reasons. It is an oxidizing agent that can directly kill many microorganisms; it decomposes upon contact with blood, releasing elemental oxygen gas (which inhibits the growth of anaerobic microorganisms); and it foams upon contact with blood, which provides a cleansing action. In the laboratory, small quantities of hydrogen peroxide can be prepared by the action of an acid on an alkaline earth metal peroxide, such as magnesium peroxide.

MgO₂(s) + 2 HCl(aq) → H₂O₂(aq) + MgCl₂(aq)

What amount of hydrogen peroxide should result when 1.26 g of magnesium peroxide is treated with 26.5 mL of hydrochloric acid solution containing 0.0736 g of HCl per mL?

What mass of which reagent is left unreacted?

Answer 1

MgO₂(s) + 2 HCl(aq) → H₂O₂(aq) + MgCl₂(aq)

Molecular weight of MgO2= 27 (atomic weight of Mg) +2*16 (molecular weight of oxygen)= 59 gm/mole

Molecular weight of HCl =1+35.5= 36.5 gm/mole and that of H2O2= 2+32= 34

Moles of MgO2= 1.26 (mass)/59 (Molecular weight)= 0.021356

Mass of HCl = 26.5 ml* 0.0736 g/mol =1.95 gms = 1.95/36.5 moles= 0.053425 moles

Molar ratio of reactants as per the reaction = MgO2 : HCl= 1:2

Given ratio of reactants = 0.021356 : 0.053 = 1:2.5

Excess reactant is HCl ( since requires is 2 and actually available is 2.5)

Since limiting reactant decides the extent of H2O2 formed,

1 moles of MgO2 produces 1 mole of H2O2

0.021356 moles produces 0.021356* moles of H2O2= 0.021356*34gms of H2O2=0.726 gms

Mass of HCl remaining = Mass of HCl supplied- HCl converted= 1.95- (2*0.021356*36.5) ( 2* moles of MgO2* molecular weight of HCl)= 0.39 gms