Science practice 58

Radium-226 (²²⁶Ra) activity is often elevated in bodies of water that have been augmented with groundwater. Samples were collected over a two-year period in order to study ²²⁶Ra activity in Florida Marsh and Cypress wetlands. Figure 1 shows the average ²²⁶Ra activity from surface sediment samples collected in 2002 and 2004. Cores were taken from shallow sediment (0 to 4 cm) in the wetlands and analyzed for ²²⁶Ra activity. The activity, in disintegration per minute per gram (dpm/g), found in the Marsh and Cypress wetlands is displayed in Figures 2 and 3.

Figure 1

Figure 2

Figure 3

The balanced chemical equation for the dissociation of acetic acid in water is shown below:

CH₃COOH(aq) CH₃COO^- (aq) + H⁺(aq)

The reaction can proceed in either direction in order to maintain a constant value for the acid dissociation constant. Acetic acid is an acid because it contributes H⁺ ions to the solution. The acid dissociation constant for the above reaction can be expressed as follows:

The expression [A] represents the concentration of substance A in moles/liter. One mole is 6.02 · 10²³ molecules.

Experiment 1

NaOH is a strong base because it dissociates readily in water and contributes OH^– ions to the water. A 1 M solution of NaOH, a strong base, is slowly added to a 100 ml, 2 M (M represents molarity, a measure of concentration in ) solution of acetic acid. The NaOH solution contributes OH^– ions to the mixture. An ion is a charged atom or molecule. The OH^– ions from the NaOH combine with the H⁺ ions from the acetic acid to form water as shown below:

H⁺(aq) + OH^– (aq) → H₂O (l)

Various measurements were taken and the results are shown in Table 1 and graphed in Figure 1. Note: pH = –log [H⁺]

Table 1

Figure 1