Hydrate Lab – Determining Water of Hydration

Equipment & Materials

• Crucible (with matching lid)
  • Handle only with crucible tongs; never with fingers.
• Crucible tongs
  • Practice gripping an empty crucible both vertically and horizontally so you can safely insert/remove it from the triangle.
• Hydrate sample (white ionic solid containing bound water)
• Clay/porcelain triangle (supports the crucible during heating)
• Ring stand with adjustable iron ring
• Bunsen burner
  • Gas outlet on the bench (blue‐labeled “GAS”) + rubber tubing
  • Burner barrel with adjustable air vents (holes) and a needle valve at the base
  • Bench gas knob that must be turned fully on during operation
  • Wooden matches or striker
• Balance (±0.001 g precision)
• Ceramic heat-resistant tile (resting pad for hot crucible; prevents bench scorching)
• Safety goggles (fully enclosed or vented; choose vented types if fogging occurs)

Safety Precautions

• Goggles must be worn at all times; each student runs an individual burner.
• Tie back hair, remove loose sleeves, keep combustibles away from flame.
• Hot crucibles look identical to cold ones—assume they are hot; use tongs.
• Never place a hot crucible directly on the benchtop; use ceramic tiles supplied.
• Do not overfill the crucible; ⅓–½ full prevents spattering.
• Keep flame small and mostly blue; large flames are unstable and hazardous.
• Ensure laboratory ventilation; proper burner operation prevents CO/soot.

Bunsen Burner Setup & Operation

• Connect tubing to the bench gas outlet and burner base.
• Bench gas knob: turn fully open when ready to ignite.
• Needle valve (burner base):
  • Start closed, then open ½–1 turn; excessive opening causes flame lift-off.
• Air vents (barrel): opened slightly for a steady, blue, inner-cone flame.
• Lighting procedure:
  1. Close air vents (reduces flashback).
  2. Hold match above barrel and turn on burner gas (if separate needle valve) or bench knob.
  3. Once ignited, open air vents until a small, sharply-defined blue inner cone appears.
  4. Regulate flame height with the needle valve, not bench knob.
• Desired flame: inner cone ~2 cm high; tip of inner cone is hottest region (≈ 800–900 °C).
• Shut-down: close bench knob first, then close burner valves.

Ring Stand Arrangement

• Adjust ring so the clay triangle is ≈ 2–3 cm (≈ 1 inch) above the burner tip.
• Triangle sits flat on the ring; crucible rests securely inside the triangular opening.
• Position burner so the flame’s inner cone touches the bottom center of the crucible.

Experimental Procedure (Overview)

1. Mass of Empty Crucible
   • Clean/dry crucible & lid; cool to room temp.
   • Weigh crucible + lid on the balance; record $m_{\text{crucible}}$.
2. Add Hydrate
   • Transfer hydrate with spatula until crucible is ⅓–½ full.
   • Re-weigh crucible + lid + hydrate; record $m_{\text{crucible + hydrate}}$.
3. Heat to Drive Off Water
   • Place crucible (lid slightly ajar) on triangle; begin gentle heating.
   • Gradually increase to full heat; maintain for ≈ 30 min to ensure complete dehydration.
   • Observation: solid remains white but water vapor escapes invisibly.
4. Cool & Weigh Anhydrous Salt
   • With tongs, move crucible to ceramic tile; cool ≥ 5 min.
   • Weigh crucible + anhydrous solid; record $m_{\text{crucible + anhydrous}}$.
5. Repeat?
   • In this lab one long heating replaces multiple shorter reheats; no second firing required unless instructed.
6. Clean-up
   • Dump cooled anhydrous solid into designated solid-waste bucket.
   • Return crucible, lid, triangle, and tongs to drawer.
   • Close gas valves.

Data Table (What To Record)

• $m_{\text{crucible}}$ (g)
• $m_{\text{crucible + hydrate}}$ (g)
• $m_{\text{crucible + anhydrous}}$ (g)
• All masses to 0.001 g (balance precision)

Core Chemical Theory

• Hydrate = ionic compound containing trapped water molecules in fixed ratio.
  • General notation: $\text{Salt}\cdot x\,\text{H}_2\text{O}$
• Heating causes dehydration:
  $\text{Salt}\cdot x\,\text{H}<em>2\text{O}</em>{(s)} \;\xrightarrow{\Delta}\; \text{Salt}<em>{(s)} + x\,\text{H}</em>2\text{O}_{(g)}$
• Law of Conservation of Mass:
  $(m<em>{\text{hydrate}} = m</em>{\text{anhydrous}} + m_{\text{water}})$
• "Anhydrous salt" = dehydrated ionic compound ("salt"), free of water.

Calculations (Front Page)

1. Mass of Hydrate
   $m<em>{\text{hydrate}} = m</em>{\text{crucible + hydrate}} - m_{\text{crucible}}$
2. Mass of Anhydrous Salt
   $m<em>{\text{salt}} = m</em>{\text{crucible + anhydrous}} - m_{\text{crucible}}$
3. Mass of Water Lost$m<em>{\text{water}} = m</em>{\text{hydrate}} - m_{\text{salt}}$
   • Example: if $m<em>{\text{hydrate}} = 10.000\,\text{g}$ and $m</em>{\text{salt}} = 7.000\,\text{g}$, then $m_{\text{water}} = 3.000\,\text{g}$.
4. Percent Water in Hydrate$\%\,\text{H}<em>2\text{O} = \frac{m</em>{\text{water}}}{m_{\text{hydrate}}} \times 100\%$
   • Physically cannot exceed $100\%$.
5. Significant Figures
   • Subtractions use decimal-place rule (least decimal place from masses).
   • Percent water uses division rule (least number of sig figs from numerator/denominator).

Calculations (Back Page – Mole Relations)

• Goal: determine $x$ in $\text{Salt}\cdot x\,\text{H}_2\text{O}$.

1. Moles of Water$n<em>{\text{water}} = \frac{m</em>{\text{water}}}{M_{\text{water}}}$
   • $M_{\text{water}} = 18.015\,\text{g\,mol}^{-1}$.
2. Moles of Anhydrous Salt$n<em>{\text{salt}} = \frac{m</em>{\text{salt}}}{M_{\text{salt}}}$
   • $M_{\text{salt}}$ supplied by instructor (formula unknown to students).
3. Mole Ratio (Equation 2 in lab manual)$\text{ratio} = \frac{n<em>{\text{water}}}{n</em>{\text{salt}}}$
   • Units can be $\frac{\text{mol H}_2\text{O}}{\text{mol salt}}$ or simplified to plain number.
4. Determine Whole-Number Hydration (x)
   • Round ratio to nearest integer (e.g.
     • 5.94 → 6
     • 2.02 → 2)
   • Final empirical formula: $\text{Salt}\cdot x\,\text{H}_2\text{O}$.
5. Sample Molar-Mass Calculation Reminder
   • For $\text{NH}_3$:
     $M = 1\,(14.01) + 3\,(1.008) = 17.034\,\text{g\,mol}^{-1}$.
   • For $\text{CO}_2$:
     $M = 1\,(12.01) + 2\,(16.00) = 44.01\,\text{g\,mol}^{-1}$.

Post-Lab & Disposal

• After recording final mass, tip anhydrous salt into white solid-waste container.
• Return crucible, lid, triangle, and tongs; leave ring stand & burner at station.
• Close bench gas valve; verify burner extinguished.
• Wipe bench; turn in data sheets.

Tips, Pitfalls & Common Errors

• Overfilling crucible → spattering & loss of sample → low water %.
• Incomplete dehydration (short heating) → residual water → calculated $m_{\text{water}}$ too low, %H₂O too small, $x$ rounds down.
• Spilled solid during weighing → erroneous low salt mass → artificially high %H₂O, $x$ too large.
• Remember: masses are differenced first; sig-fig mistakes often appear in the % calculation and mole ratio.
• Do not confuse molar mass (g mol⁻¹) with moles (mol).

Connections & Real-World Relevance

• Hydrates are prevalent (e.g.
  plaster of Paris $\text{CaSO}<em>4\cdot ½\,\text{H}</em>2\text{O}$; Epsom salt $\text{MgSO}<em>4\cdot 7\,\text{H}</em>2\text{O}$).
• Dehydration/rehydration cycles used in desiccants, concrete curing, chemical storage.
• Law of Conservation of Mass underpins balancing of all chemical equations and stoichiometric scaling.
• Percent composition by mass is foundational for quality control in pharmaceuticals, food, and materials science.