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 $\ce{6H2O}$?

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    $\begingroup$ 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. $\endgroup$ – Zhe Oct 19 '16 at 21:20
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    $\begingroup$ You can edit extra information into your question, so you don't need to add extra comments. $\endgroup$ – orthocresol Oct 19 '16 at 21:21
  • $\begingroup$ So would the mass percent calculated by this site be wrong then? webqc.org/molecular-weight-of-NH4MgPO4*H2O.html $\endgroup$ – Siddart Fredrick Oct 19 '16 at 21:21
  • $\begingroup$ It's magnesium ammonium phosphate hexahydrate, aka struvite (the mineral struvite includes the hexahydrate part). $\endgroup$ – airhuff Feb 16 '17 at 9:48

As stated in one of the comments to your question, the waters of hydration constitute part of the crystal structure itself, just as much so as the rest of the molecule. You can somewhat think of the waters as being functional groups of the compound. For example, the dehydrated form of a compound can have very different chemical and physical properties than the hydrated form. The presence of waters of hydration in no way means that the compound is aqueous or dissolved in water.

Thus, when calculating the mass percentage of hydrogen of the hydrated compound, you do include the hydrogens from both the waters of hydration and the rest of the molecule.

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.

Here is a photograph of struvite crystals from the same article:

enter image description here


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