Can I use tartaric acid in making Snickerdoodles?
I've never tried it, but you might try 1/2 the amount of tartaric acid based on information from Nigella Lawson :

Cream of tartar is made by combining tartaric acid with potassium hydroxide. This partially neutralizes the tartaric acid, so cream of tartar is less acidic than tartaric acid.
If the tartaric acid is used in baking or added to egg whites before whisking into meringues then it should be possible to use cream of tartar in roughly double the quantities of the tartaric acid, though we have not tested this. However we would not recommend using cream of tartar as a substitute in any other recipe.