Question

Copper metal (Cu) reacts with silver nitrate (AgNO3) in aqueous solution to form Ag and Cu(NO3)2. An excess of AgNO3 is present. The balanced chemical equation is shown below.
Cu + 2AgNO3 mc021-1.jpg Cu(NO3)2 + 2Ag

The molar mass of Cu is 63.5 g/mol. The molar mass of Ag is 107.9 g/mol. What mass, in grams, of Ag is produced from reaction of 31.75 g of Cu?
a.26.95
b.107.9
c.215.91
d.431.82

Answer 1

Answer : The correct option is, (B) 107.9 grams

Explanation :

First we have to calculate the moles of $Cu$.

$\text{Moles of }Cu=\frac{\text{Mass of }Cu}{\text{Molar mass of }Cu}=\frac{31.75g}{63.5g/mole}=0.5moles$

Now we have to calculate the moles of $Ag$.

The balanced chemical reaction is,

$Cu+2AgNO_3\rightarrow Cu(NO_3)_2+2Ag$

From the balanced reaction we conclude that

As, 1 mole of $Cu$ react to give 2 mole of $Ag$

So, 0.5 mole of $Cu$ react to give $0.5\times 2=1$ mole of $Ag$

Now we have to calculate the mass of $Ag$

$\text{Mass of }Ag=\text{Moles of }Ag\times \text{Molar mass of }Ag$

$\text{Mass of }Ag=(1mole)\times (107.9g/mole)=107.9g$

Therefore, the mass of Ag produced is, 107.9 grams

Answer 2

Stoichiometry time! Remember to look at the equation for your molar ratios in other problems.

31.75 g Cu | 1 mol Cu | 2 mol Ag | 107.9 g Ag         6851.65
⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻   →   ⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻⁻  = 107.9 g Ag
       ∅        | 63.5 g Cu | 1 mol Cu | 1 mol Ag              63.5


There's also a shorter way to do this: Notice the molar ratio from Cu to Ag, which is 1:2. When you plug in 31.75 into your molar mass for Cu, it equals 1/2 mol. That also means that you have 1 mol Ag because of the ratio, qhich you can then plug into your molar mass, getting 107.9 as well.