**Step 1:**** Calculate the mass and moles of MgSO _{4} and H_{2}O in the compound.**

*The problem states that the sample of Epsom salt is heated to drive off the water of hydration:*

**mass of hydrated (with water) Epsom salt = 4.93 g**

**mass of dehydrated (without water) Epsom salt = 2.41 g MgSO _{4}**

Therefore, the mass of water is then the difference in mass:

**mass of water = ****mass of hydrated Epsom salt ****– ****mass of dehydrated Epsom salt**

**mass of water = 4.93 g**** ****– 2.41 g**

**mass of water = 2.52**** g H _{2}O**

Now, we need to get the moles of each MgSO_{4} and H_{2}O compound. We will need their molar mass for the calculation:

molar mass MgSO_{4}:

**MgSO**** _{4}** 1 Mg x 24.3 g/mol = 24.3 g/mol

1 S x 32.07 g/mol = 32.07 g/mol

4 O x 16.00 g/mol = 64.00 g/mol

_____________________________

molar mass H_{2}O:

**H**** _{2}O** 2 H x 1.008 g/mol = 2.016 g/mol

1 O x 16.00 g/mol = 16.00 g/mol

____________________________

*mass (molar mass) → moles*

$\mathbf{moles}\mathbf{}{\mathbf{MgSO}}_{\mathbf{4}}\mathbf{=}\mathbf{2}\mathbf{.}\mathbf{41}\mathbf{}\overline{)\mathbf{g}\mathbf{}{\mathbf{MgSO}}_{\mathbf{4}}}\mathbf{\times}\frac{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}{\mathbf{MgSO}}_{\mathbf{4}}}{\mathbf{120}\mathbf{.}\mathbf{37}\mathbf{}\overline{)\mathbf{g}\mathbf{}{\mathbf{MgSO}}_{\mathbf{4}}}}$

**moles MgSO _{4}**

$\mathbf{moles}\mathbf{}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{=}\mathbf{2}\mathbf{.}\mathbf{52}\mathbf{}\overline{)\mathbf{g}\mathbf{}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}}\mathbf{\times}\frac{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}}{\mathbf{18}\mathbf{.}\mathbf{016}\mathbf{}\overline{)\mathbf{g}\mathbf{}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}}}$

**moles H _{2}O **

**Step 2:**** Determine the lowest whole number ratio of MgSO _{4} and H_{2}O to get the formula of the compound**

Epsom salts is a hydrated ionic compound with the following formula: MgSO_{4} • xH_{2} O. A 4.93 g sample of Epsom salts is heated to drive off the water of hydration. The mass of the sample after complete dehydration was 2.41 g.

Find the number of waters of hydration (x) in Epsom salts.

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