Citric Acid crystals?

I have a formula that calls for Citric Acid (Crystal) in 40g. I only have Citric Acid Anhydrous. How can I convert this?

From Wikipedia,
Molar mass is
192.124 g/mol (anhydrous)
210.14 g/mol (monohydrate)

So in most applications like stop baths, they are close enough. Otherwise, you can convert 40 g of citr.acid monohydr. to mols: 40 g / (210.14 g/mol) = 0.1903 mol, and then, this to citr. acid anhydrous back to grams, 0.1903 mol * 192.124 g/mol = 36.57 g.

Or, you can calculate a ratio of molar weigths to get a conversion multiplier, or find such a multiplier from somewhere, I'm sure there are websites that list common conversion factors for most photographic chemicals.

Common substitutions are different amount of water in crystal and substituting Na salt for K salt or vice versa.

The amount of the anhydrous material will be roughly 4 percent less than the equivalent amount of the monohydrate, according to the above. That should be close enough for just about any photographic use.

Anhydrous just means "without water" in the crystal structure. If you have a monohydrate (or XXXhydrate) you can generally create the anhydrous version by heating it up to drive off the water of crystallisation.
The only thing I can thing that you might need citric acid for in photographic terms is stop bath, which is not a critical step and just needs to be "a bit acidic" to kill the developer's action (which will only work in alkali conditions).
If I really want a stop bath (unlikely) the I just put a couple of spoons of citric acid into a litre of water, or a slosh of white vinegar.

The only thing I can thing that you might need citric acid for in photographic terms is stop bath

Click to expand...

There are several other formulae that call for citric acid besides the stop bath.
For instance, I use an amidol developer with citric acid, where the amount of acid is important. Too little, and the developer has a very short usable life. Too much, and the pH gets too low for the developer to work.
Some toners use citric acid, too. I sometimes use an iron blue toner where the amount if citric acid is important. Too little, and and toner doesn't work. Too much, and you get ugly stains on the print.

But I think the difference in molar weight between anhydrous and monohydrate citric acid is negligible for all practical purposes.