Question

How many grams of NaOH is required to react with 3.1g of ${{P}_{4}}$ to form $P{{H}_{3}}$ and $Na{{H}_{2}}P{{O}_{2}}$?
(A) 4g
(B) 3g
(C) 2g
(D) 1g

Answer 1

The above question is based on the stoichiometry of reactions. In order to proceed with this question, we have to find how many moles of ${{P}_{4}}$ is reacting with how many moles of NaOH and by this we can find the gram of NaOH required to react with a given weight of ${{P}_{4}}$ molecule.

Complete step by step answer:
First of all, let’s get some basic idea about stoichiometry.
Stoichiometry is a branch of chemistry which deals with the calculation of reactants and products in chemical reactions. Stoichiometry is based on the law of conservation of mass which states that the total mass of the reactant is equal to the total mass of the products, leading to the relations among quantities of reactants and products. This means that if the amounts of the individual reactants are known to us, then we can calculate the amount of the product. Conversely, if one reactant is of a known quantity and the quantity of the products can be determined empirically, then the amount of rest of the reactants can also be calculated.
For example, in the reaction between methane and oxygen, carbon dioxide and water are formed as a product. Its equation can be written as follows
$C{{H}_{4}}+2{{O}_{2}}\to C{{O}_{2}}+2{{H}_{2}}O$
In the above reaction, one molecule of methane gas reacts with two molecules of oxygen gas to produce one molecule of carbon dioxide and two water molecules. The above chemical reaction is an example of the combustion reaction. Stoichiometry measures those quantitative relationships and then uses it to determine the number of products and reactants that are produced or needed in a given reaction to getting the desired amount of product. Describing the quantitative relationships among substances as they participate in a reaction is known as reaction stoichiometry. In the above example, reaction stoichiometry measures the relationship between the quantities of methane and oxygen that should react to form the desired amount of carbon dioxide and water.
Now, we have a basic knowledge of reaction stoichiometry. To find our answer, first, we have to write the balanced chemical equation of the reaction which is as follows:
${{P}_{4}}+3NaOH+3{{H}_{2}}O\to P{{H}_{3}}+3Na{{H}_{2}}P{{O}_{2}}$
Given, mass of ${{P}_{4}}$ = 3.1gram
We know, molecular mass of ${{P}_{4}}$ = 124 g/mol
Then, number of moles of ${{P}_{4}}=\dfrac{3.1}{124}=0.025mol$
By looking at the chemical reaction, it is clear that 3 mol of NaOH reacts with 1 mol of ${{P}_{4}}$.
So, 0.025 mol of ${{P}_{4}}$ reacts with $3~\times 0.025\text{ }=\text{ }0.075\text{ }mol$ of NaOH.
Now, the molecular mass of NaOH = 40 g/mol
So, mass of 0.075 mol of NaOH = $40\times 0.075=3g$
Hence, 3g of NaOH is required to react with 3.1g of ${{P}_{4}}$.

Therefore, the correct answer is an option (B), 3g.

Note: If the amount of any one of the reactants or products is known then the amount of rests of the reactants or products can be calculated by using stoichiometry but you have to make sure that the reaction is in the balanced form.