"Doesn't match."
Toma stared at the calculation sheet.
"What doesn't?" Kana asked.
"Mixed reagents, but product doesn't match theory."
Rei approached. "Did you check stoichiometry?"
"Stoichiometry?"
"Coefficients in reaction equation. Balance to match atom numbers."
Milia explained. "Law of conservation of mass. Atoms don't disappear or newly appear."
"Isn't that obvious?" Kana said.
"Obvious, but this is foundation of everything," Rei emphasized.
Toma wrote reaction equation. "2H2 + O2 → 2H2O"
"Two hydrogen molecules and one oxygen molecule make two water molecules."
Rei nodded. "Coefficients are stoichiometric ratio. Without following this, there's waste."
Kana wrote in her notebook. "Waste?"
"Limiting reagent. The one that runs out first."
Milia gave example. "If you mix one hydrogen molecule with one oxygen molecule?"
"Water?" Toma thought.
"Can make two molecules, but not enough hydrogen. Only one water molecule."
"Oxygen left over?"
"Half remains. Called excess reagent."
Rei continued. "That's why mixing in stoichiometric ratio is important."
Kana asked. "But in Toma's experiment?"
"Mixed in stoichiometric ratio, but product is less."
Milia asked. "What percentage of theoretical yield?"
"About 60 percent."
"Typical. Almost no reactions are perfect."
Toma was surprised. "Not perfect?"
"Side reactions, reverse reactions, losses... various factors," Rei explained.
"Side reactions?"
"Reactions forming products other than intended. Selectivity isn't 100 percent."
Kana understood. "That's why yield decreases?"
"Yes. Experimental yield is always lower than theoretical yield."
Milia added. "Same with biochemical reactions. Even with enzymes, not 100 percent."
"Even with enzymes?"
"Reverse reactions occur. Because there's equilibrium."
Rei emphasized. "Stoichiometry tells maximum. But doesn't guarantee."
Toma murmured. "Contradictions stoichiometry won't allow?"
"Reaction equations where atom numbers don't match," Milia answered.
"Example?"
"H2 + O2 → H2O. Three atoms on left, three on right. But hydrogen and oxygen numbers don't match."
Kana noticed. "Coefficients needed!"
"Yes. 2H2 + O2 → 2H2O. Now both sides match."
Rei continued. "Law of conservation of mass is absolute. Reactions violating this don't exist."
"Except nuclear reactions," Milia added.
"Nuclear reactions?"
"Mass converts to energy. E=mc²."
Toma got excited. "In chemical reactions, mass is invariant?"
"Strictly, changes very slightly. But too small to measure."
Kana looked at calculation sheet. "Toma's experiment, stoichiometrically correct?"
Rei checked. "Coefficients match. But might be experimental technique issue."
"Technique?"
"Temperature, pH, stirring, purity... many factors affect yield."
Milia encouraged. "60 percent isn't bad. Even pros, over 80 percent is excellent."
Toma laughed. "Really?"
"In organic synthesis, multi-step reactions sometimes below 10 percent."
Kana was impressed. "Chemistry has big gap between theory and practice?"
"That's why it's interesting," Rei said. "Theory shows upper limit, experiment teaches reality."
Milia continued. "But stoichiometry never lies."
"Absolute?"
"Absolute. Atoms don't disappear. Don't increase. Only move."
Toma took deep breath. "Then where's my 40 percent?"
"Byproducts, losses, measurement error," Rei enumerated.
"Can I find if I search?"
"Theoretically. But actually difficult."
Kana murmured. "Stoichiometry, strict but fair."
"Has mathematical beauty," Milia acknowledged.
Rei added. "This rigor made chemistry a science."
Toma laughed. "Won't allow contradictions."
"Won't allow. But that's why trustworthy."
Kana looked at calculation sheet again. "Numbers speak truth."
"Stoichiometry is nature's accounting," Milia said quietly.
"Must balance."
Rei nodded. "That's chemistry's aesthetic."
The four stared at the calculation sheet. In numbers, immutable truth exists.