Читать книгу Food Chemistry - Dennis D. Miller - Страница 24

1 The Ka for the weak acid HA is 4.0 × 10−6. What is the pH of a 0.01 M solution of this acid? What is its pKa?

2 How many grams of acetic acid and sodium acetate are required to prepare 1.0 l of 0.5 M acetate buffer, pH 4.5? The pKa for acetic acid is 4.75.

3 Explain carefully how to prepare 1.0 l of 0.05 M phosphate buffer, pH 7.0 from NaH2PO4 .H2O and 1.0 N NaOH or 1.0 N HCl. The molecular weight of sodium phosphate monohydrate monobasic is 138 g mole−1. The pKa2 for H3PO4 is 7.21. Hint: To determine whether you will need to add NaOH or HCl, you need to calculate the pH of a 0.05 M solution of NaH2PO4.

4 Preparation of buffers using published tables.Using Tables III.2a, III.2b, and III.2c in Appendix III, describe carefully how you would prepare 1 l of 0.1 M acetate buffer, pH 5.2 and 1 l of 1/15 M phosphate buffer, pH 7.6.Using the Henderson–Hasselbalch equation (shown below), calculate the theoretical pH of these 2 buffers. Explain why the calculated pHs are not exactly the same as the pHs shown in the Appendix table.