Final answer:
In a perfectly competitive housing construction industry, a wage increase for carpenters negotiated by a labor union can lead to an excess supply of workers at the firm level. To mitigate unemployment, the union can launch a 'Buy union' advertising campaign to increase the demand for labor.
Step-by-step explanation:
In a perfectly competitive housing construction industry, where the labor and output markets are perfectly competitive, if a labor union negotiates a wage increase for carpenters from $15 to $20 per hour, it would lead to an excess supply of workers. With the wage increase, the labor demand curve for the individual firm would shift upward, resulting in a surplus of labor because the higher wage would cause more workers to be willing to work in the industry than the firms require.
To mitigate the unemployment caused by the wage increase, the union can bolster demand by launching a 'Buy union' advertising campaign. If the union spends $5 million on the campaign, it would increase the demand for labor, leading to a decrease in the excess supply of labor.