Question

An aqueous solution containing \(10.0 \mathrm{~g} \mathrm{NaOH}\) is added to an aqueous solution containing \(10.0 \mathrm{~g} \mathrm{H}_{3} \mathrm{PO}_{4}\). (a) Write a balanced equation for the acid-base reaction that should occur. (b) Identify the limiting reactant. (c) Will the solution be acidic or basic when the reaction is complete?

Short answer

The balanced equation for the reaction is \(3NaOH + H3PO4 -> Na3PO4 + 3H2O\). The limiting reactant is the NaOH. The final solution will be acidic because not all of the phosphoric acid will react.

Step-by-step solution

Write Down a Balanced Equation

The reaction between the sodium hydroxide (NaOH), which is a base, and the phosphoric acid (H3PO4), which is an acid, can be represented as follows:\n\n\(3NaOH + H3PO4 -> Na3PO4 + 3H2O\)\n\nThis is an example of an acid-base neutralization reaction where an acid reacts with a base to form water and a salt. In this specific case, the salt is sodium phosphate (Na3PO4).

Identify the Limiting Reactant

The limiting reactant is the substance that is completely consumed in a chemical reaction and determines when the reaction will stop. To identify the limiting reactant, first, calculate the number of moles of each reactant:The molar mass of NaOH is approximately 40 g/mol and for H3PO4 is approximately 98 g/mol. Thus, The number of moles of NaOH = \( \frac{10.0 g}{40 g/mol} = 0.25 mol \)The number of moles of H3PO4 = \( \frac{10.0 g}{98 g/mol} = 0.102 mol \)From the balanced chemical equation, it is clear that the reaction requires 3 moles of NaOH for every mole of H3PO4. However, we don't have 3 times as many moles of NaOH than H3PO4, therefore, NaOH is our limiting reactant.

Determine the Nature of the Solution

As the limiting reactant is NaOH, not all of the phosphoric acid (H3PO4) will react. This means that there will be some unreacted phosphoric acid left in the solution. Since phosphoric acid is an acid, this means that the solution will remain acidic when the reaction is complete.

Key concepts

Limiting Reactant

Understanding the concept of a limiting reactant is essential when studying chemical reactions. In every chemical process, the quantities of reactants present can determine the amount of products formed. The limiting reactant is the substance that is completely used up first during a chemical reaction, thus dictating the maximum amount of product that can be created. This concept is similar to a recipe; if you're baking cookies and run out of flour, you can't make more cookies, regardless of how much sugar or butter you have left.

To find the limiting reactant in any reaction, start by calculating the number of moles of each reactant. Then, use the balanced chemical equation to compare these mole ratios to the coefficients in the equation, representing the stoichiometric ratio required for the reaction to proceed. In the provided exercise, sodium hydroxide (NaOH) is identified as the limiting reactant because there is not enough of it to fully react with the phosphoric acid (H3PO4) present. Recognizing the limiting reactant is crucial for predicting the outcome of reactions and optimizing the use of materials in industrial processes.

Chemical Stoichiometry

Chemical stoichiometry refers to the quantitative relationship between reactants and products in a chemical reaction. It is based on the conservation of mass and the concept that elements are neither created nor destroyed during a chemical reaction. Hence, stoichiometry allows chemists to predict how much product will form from a given amount of reactants or how much of the reactants are required to produce a desired quantity of product.

By using the balanced equation of an acid-base neutralization reaction, for example, one can determine the stoichiometry of the substances involved. In the exercise, the chemical reaction between NaOH and H3PO4 is considered. According to the balanced equation, three moles of NaOH react with one mole of H3PO4 to produce one mole of sodium phosphate (Na3PO4) and three moles of water (H2O). Stoichiometry is critical in all areas of chemistry, from the small-scale laboratory to large-scale industrial processes, because it ensures efficient use of resources and helps in environmental conservation by minimizing waste.

Properties of Acids and Bases

Acids and bases have distinct properties which are often studied in chemistry. Acids are substances that can donate a hydrogen ion (H+) when dissolved in water, and they generally have a sour taste, can corrode metals, and turn blue litmus paper red. Examples include citric acid in citrus fruits and hydrochloric acid in stomach digestion. Bases, on the other hand, can accept a hydrogen ion and often have a bitter taste and slippery feel; they turn red litmus paper blue. Common bases include baking soda (sodium bicarbonate) and ammonia.

In an acid-base neutralization reaction, an acid reacts with a base to form a salt and water. Neutralization typically results in a salt solution with a pH closer to 7. However, if either the acid or base is in excess, as in the case of the provided exercise, the resulting solution will have a pH indicative of the excess component. Since NaOH is the limiting reactant and not all of the H3PO4 will react, the remaining acidic solution will display the characteristic properties of an acid, such as lowering the pH below 7.