ACIDS AND BASES:

STANDARDIZATION and ANALYSIS

Learning Objectives

Upon completion of this experiment, students will have experienced:

1. 1. 1. Predict and balance the reactants and products in an acid - base reaction

      2. Calculate the mass, mole and molarity of each species within the balanced chemical reaction.

      3. Use a primary standard to titrate a solution and determine its molarity

      4. Use a standardized solution to determine the molar mass of an unknown substance.

Discussion

Acids by definition are substances that generally contain a hydrogen atom that when placed in water becomes a hydrogen ion, H⁺, in solution. When a compound creates charge in an aqueous solution, it is also called an electrolytic solution. By the definition of Brönsted-Lowery, an acid is a hydrogen ion donator; therefore, it increases the hydrogen ion in solution, this does not mean that an acid has to contain a hydrogen atom. A strong acid is also a strong electrolyte. Several examples of common strong acids are HCl, HNO₃, H₂SO₄, HClO₄ and H₃PO₄. A base can be defined in two ways, first as a substance that contains the hydroxide ion, OH^1-, or by Brönsted-Lowery, a substance that accepts a hydrogen ion and therefore, increases the hydroxide ion concentration in solution. Examples of strong bases include, NaOH, KOH. Examples of weak bases are NH₃ and CO₃^2- ion. Salts are electrolytes, which are ionic compounds where, in general, a metal bonds with a non-metal or polyatomic ion. We can also look at a salt as the product of an acid - base reaction.

When an acid reacts with a base, a double replacement reaction occurs and the products are water and a salt.

Acid + Base ⇄ Salt + Water

Examples of an acid base reaction include:

HCl + NaOH ⇄ NaCl + HOH Reaction 1

2 H₃PO₄ + 3 Ca(OH)₂ ⇄ Ca₃(PO₄)₂ + 6 H₂O Reaction 2

When the moles of H⁺ = moles of OH^1-, this is called the equivalence point. The second reaction has up to 6 equivalence points and is a very difficult problem to solve. In this lab we will use a monoprotic acid, which has only one equivalence and the strong base, NaOH, which also has only one equivalence. Therefore the equivalence point is the stoichiometric point at which the moles of acid = moles of NaOH.

The difficulty in this reaction is how we determine the equivalence point. This point can be calculated but when using an unknown, we need to see this point visually. Therefore, we use an indicator that when the pH of the solution changes, due to the changing hydrogen ion concentration, the indicator changes color and allows a visual indication of the endpoint, which if the indicator is chosen correctly is with very little error, can be considered to be the same as the equivalence point. In a titration of an acid with a base, the indicator shows the end point, at which point we can read a volume of the titrant and use this volume to determine the moles at the equivalence point.

Pre laboratory assignments

Complete the pre laboratory writing assignment in your laboratory notebook. The prewriting must be completed before the laboratory session begins. The writing should include a purpose/hypothesis of the experiment, a discussion or theory, a detailed step by step summary of the procedure explaining what data will be collected and how you will collect it. Data Tables or space to collect the data should be included within the procedure discussion. Look up any SDS on chemicals that you are not familiar.

Complete the pre laboratory problems.

Procedure

Part I. Indicators

Use the pictures below to describe the color changes that occur in the given indicators based upon the pH of the solution. Set up 30 test tubes. To 6 test tubes add approximately 1 mL of 0.01 M HCl. To a second set of 6 test tubes add approximately 1 mL of 0.01 M NaOH. Add approximately 1 mL of the pH 4, 7 and 10 buffer solutions into 6 test tubes each. To a set of test tubes that contain each solution, add 5 drops of each indicator solution.

Part II. Standardization of sodium hydroxide solution

Report and Conclusion Paragraph

Complete the data and result tables, graphs, calculations and answer all required questions for Parts I and II for this part of the combined experiment.

Complete a conclusion paragraph using the RERUNS method for the combined experiment 30/31/32. A discussion of how to write a conclusion paragraph is given in Appendix D: How to Write a Formal Laboratory Report