A: The humidity ratio has units of lb_water / lb_dry-air, so multiplying by density (lb_air / ft³) correctly gives lb_water / ft³ — and then multiplying by volume gives lb_water. The issue is likely in how you're using the humidity ratio or specific volume from the psychrometric chart: make sure you're pulling the right value for that state and watch the unit chain carefully. Also, using a standard density of 0.075 lb/ft³ may introduce error if the actual air conditions are far from standard — use the actual specific volume when it's available.