But how do I get the baking soda in my inventory?
You need to have completed the mansion and talked to jason about what to do next. To do that you have to open the backroom in your house. The key is in your backyard, behind a pushable crate so you need to find the push skill in the haunted house.
That skill can be found in both way, left or right depend of your maturity level.
Hey,
The backroom being open or not shouldn't prevent you for continuing the game. If you're lost, don't hesitate to check the walkthrough here ;
https://board.net/p/PaciProjectWalkthrough
Cheers