Thanks for the question. Powdered sugar is simply granulated sugar that has been crushed into a fine powder. Cornstach is often added to powdered sugar to prevent clumping. You can make powdered sugar from granulated sugar by simply blending 1 cup granulated sugar and 1 teaspoon cornstarch until a fine powder.
1 3/4 cup powdered sugar can be substituted for 1 cup granulated sugar but the sucess of the recipe really depends on how you are using the sugar. If it is in a baked good, you should be ok but if you are trying to make a sweet sauce then the starch in the powdered sugar may cause it to thicken more quickly then you would like.
Answered 06/04/2013 by kolson