Question

An alkaline battery produces electrical energy according to the following equation.
Zn(s) + 2 MnO2(s) + H2O(l) Zn(OH)2(s) + Mn2O3(s)
(a) Determine the limiting reactant if 17.5 g Zn and 31.0 g MnO2 are used.
(Type your answer using the format CH4 for CH4.)
(b) Determine the mass of Zn(OH)2 produced. _ g

Answer 1

a) Given reaction:

Zn + 2MnO2 + H2O → Zn(OH)2 + Mn2O3

1 mole of Zn combines with 2 moles of MnO2

Now:

# moles Zn present = 17.5/65.38 = 0.2677 moles

# moles of MnO2 present = 31.0/86.94 = 0.3566 moles

Since # moles of MnO2 is less than Zn i.e. it is not present in the 2:1 ratio (MnO2:Zn), MnO2 will be the limiting reagent

b) Based on stoichiometry:

2 moles of MnO2 produces 1 mole of Zn(OH)2

Thus, moles of Zn(OH)2 produced form the limiting reactant = 0.3566/2 = 0.1783 moles

Mass of Zn(OH)2 = 0.1783*99.42 = 17.73 g

Answer 2

Alkali battery are primary batteries that are disposable. The limiting reactant is $\bold{MnO_2}$  and the mass of $\bold{ Zn(OH)_2 }$ is 17.73 g.

What are alkali batteries?

Alkali batteries are a type of primary battery that derives its energy from the reaction between zinc and manganese oxide.

Given,

The reaction is:

$\bold{Zn + 2MnO_2 + H_2O = Zn(OH)_2 + Mn_2O_3}$

Mass of Zn is 17.5 g and $\bold{MnO_2}$ is 31.0 g

Molar mass of:

Zn is 65.38

$\bold{MnO_2}$ is 0.3566

Step1:To calculate the number of moles

$\bold{Number\;of \;moles= \dfrac{mass}{molar\;mass}} \\\\\bold{Number\;of \;moles\;of\;Zn = \dfrac{17.5g}{65.38} =0.2677\;moles} \\\\\bold{Number\;of \;moles\;of\; \bold{MnO_2} = \dfrac{31.0g}{86.94}=0.3566}$

The moles of $\bold{MnO_2}$ is less than Zn. Thus, the limiting agent will be $\bold{MnO_2}$.

Step2: Mass of $\bold{ Zn(OH)_2 }$

From stoichiometry

2 moles of $\bold{MnO_2}$ produces 1 mole of $\bold{ Zn(OH)_2 }$

Thus, the moles of $\bold{ Zn(OH)_2 }$ produced by the limiting reactant will be mole of $\bold{MnO_2}$ will be divided by 2.

$\dfrac{0.3566}{2 } = 0.1783$

$\bold{Mass\; of\; Zn(OH)_2 = 0.1783\times99.42 = 17.73 g}$

Thus, the limiting agent is $\bold{MnO_2}$ and mass of $\bold{ Zn(OH)_2 }$ is 17.73 g.

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