Question

If the masses of $Cr$ and $O$ are in the ratio $13:12$ in $Cr{O}_3$, then the ratio of $O$ that combines with the same mass of Cr in ${Cr}_2{O}_3$ and ${Cr}_2{O}_3$ is:

• A. $1:2$
• B. $1:4$
• C. $3:7$
• D. $2:5$

Answer 1

The correct option is C $3:7$

First we will use a mass of oxygen of 12 g.

The reaction of oxygen and Chromium to from dichromate $Cr{O}_7$ is

$2Cr+7O_2$ $\to$ $2Cr{O}_7$

Mole ratio is 2:7:2

In this case 13 g of Cr will be used in this equation.

Calculating the moles of Cr (molar mass = 52)

moles = mass/molar mass

= 13/52

= 0.25 moles

mole ratio between $Cr:O_7$ is 2:7

Therefore the moles of oxygen is 0.875, calculate the mass using this mole value.

mass = moles × molar mass

= 0.875 × 28 = 28 g

The ratio of the oxygen in the 2 equations then is:

12 g: 28 g

the ratio, therefore, is 3:7