Unblock a drain after using too much baking soda
A weak acid is the way to deal with a plug of baking soda. It'll turn the plug material into CO2 and a soluble sodium salt of the acid.
It'll take quite a bit of acid to deal with a pound of bicarb though, 84 grams is 5.4 moles of the stuff.
Regular vinegar runs at 5% acid strength, that's abot 200 grams per gallon, 3.3 moles. So you'd need abot 1.6 gallons to deal with a pound of impacted bicarb. A single gallon might do the trick though, as all you need to do is destabilize the plug, at which point the rest will wash through.

Citric acid would be a nice choice. It's easy to get, but with a molecular mass of 192, and a price around $6 for 200 grams (~1 mol), it's too expensive.

Finally, if your plumbing will take it, and you're comfortable around strong acids, hydrochloric acid (muriatic acid) would do the job. Muriatic can be had cheap at any good hardware store. Since it comes at a concentration of about 10 molar, it'll only take about 500 ml to take care of a pound of bicarb. At full strength, it'll foam and heat and boil while acting on the carbonate, so dilute with at least a gallon of water first to keep things manageable; and only try this approach if you're OK with working with around nasty, smelly, dangerous, irritating chemicals.

Trying to use lye, Drano or some other basic drain cleaner on this problem is a futile pursuit. Bicarb responds to acids, not bases.