# Do I have to consider the water in the hydrate of a salt in a mass percent calculation?

When calculating the mass percent of an element in a compound, I understand that you divide the molar mass of the element by the molar mass of a compound.

In a case such as $\ce{NH4MgPO4.6H2O}$ is the "$\ce{6H2O}$" part of the compound? If asked to find the mass percent of Hydrogen in this, what number would you use.

As well, I don't fully understand what the $\ce{6H2O}$ in the molecular formula means? Does it just mean that it is aqueous?

Any clarification would be helpful!

As well, what would be the name of that compound? Would it just be Ammonium Magnesium Orthophosphate? Or would it be something else because of the $\ce{6H2O}$?

• You're looking at a hydrate, and those are water molecules that are trapped in the spaces in the crystal structure. If you're performing stoiciometric computations using the hydrate, then yes, you should include the hydrogens in the waters of crystallization. – Zhe Oct 19 '16 at 21:20
• You can edit extra information into your question, so you don't need to add extra comments. – orthocresol Oct 19 '16 at 21:21
• So would the mass percent calculated by this site be wrong then? webqc.org/molecular-weight-of-NH4MgPO4*H2O.html – Siddart Fredrick Oct 19 '16 at 21:21
• It's magnesium ammonium phosphate hexahydrate, aka struvite (the mineral struvite includes the hexahydrate part). – airhuff Feb 16 '17 at 9:48

The common name of $\ce{NH4MgPO4⋅6H2O}$ is magnesium ammonium phosphate or struvite. According to this Wikipedia article:
Struvite (magnesium ammonium phosphate) is a phosphate mineral with formula: $\ce{NH4MgPO4·6H2O}$. Struvite crystallizes in the orthorhombic system as white to yellowish or brownish-white pyramidal crystals or in platey mica-like forms. It is a soft mineral with Mohs hardness of $1.5$ to $2$ and has a low specific gravity of $1.7$. It is sparingly soluble in neutral and alkaline conditions, but readily soluble in acid. Struvite urinary stones and crystals form readily in the urine of animals and humans that are infected with ammonia-producing organisms.