Question

Calculate the number of millimoles and milliequivalents of $C{{r}_{2}}O_{7}^{2-}$ ion in acid medium when $100~mL~$ of $0.01M~C{{r}_{2}}O_{7}^{2-}~$is reduced to $C{{r}^{3+}}~$ by $F{{e}^{2+}}.$

Answer 1

We know that the solution for the given problem depends on the basic knowledge of stoichiometry and molarity. Knowing about these topics and properly balancing the chemical equation, we can solve the given illustration.

Complete answer:
Let us know about some of the basic terms before solving the given illustration; balancing the chemical equation is the primary step before analyzing any chemical reaction and concluding something out of it. Stoichiometric coefficients: These are the coefficients used to balance the equation. We change the stoichiometric coefficients while we balance the equation, not the subscripts of the compounds involved.

Molarity: Molarity is defined as the moles of solute per litre of solution. In short, it is the amount of the substance in a certain volume of solution.
$\underset{1mol}{\mathop{C{{r}_{2}}O_{7}^{2-}}}\,+14{{H}^{+}}+\underset{6mol}{\mathop{6{{e}^{-}}}}\,\xrightarrow{{}}2C{{r}^{3+}}+7{{H}_{2}}O.$
Thus, $0.01M~C{{r}_{2}}O_{7}^{2-}~\equiv 0.06N~C{{r}_{2}}O_{7}^{2-}~$
Hence, Number of millimoles $=M\times V=0.01\times 100=1$

Therefore, Number of milliequivalents$=NV=0.06\times 100=6$

Additional Information: There are many ways to express concentration of a solution- molarity, molality, normality, mass percentage, volume percentage etc. Normality is another term used for expressing the concentration of a solution. As per the standard defined as the number of mole or gram equivalent of solute present in one liter of solution. It is denoted by ‘N’. In simple words, normality is equal to molality divided by valency. Or we can say that normality is the number of moles of solute present in equivalent weight of solution.

Note: Remember to use proper units while solving the illustrations as above. Any mistake while calculating will result in chaos which is undesirable. These are interconvertible by changing the $pH$ of the solution. Also, you should remember that in an alkaline solution, chromate ions are present while in an acidic solution, dichromate ions are present.