I'm making a dark chocolate cake, it calls for white sugar but I have an unopened bag of granulated suagr at home. Can I use that instead to save some money or will I have to buy white sugar?
If the sugar you have is actually coarser than "normal" white sugar (say, crystals are more than a mm in size):
If the recipe calls for dissolving the sugar (eg by vigorously stirring it into the wet ingredients), there is no difference at all - just make sure you dissolve it fully, coarser sugar takes more time/work to dissolve.
If the recipe calls for mixing the sugar with the fat or with dry ingredients first, sugar grain size makes a difference in texture - which will likely not ruin the cake though. If you want to be sure, you can make the sugar finer using any of a (dry!!!) blender, coffee grinder, food processor, mortar and pestle...