How do you know a chemical reaction has taken place?

If 3.5 g of element X reacts with 10.5 g of element Y to form the compound XY. what is the percent by mass of element X in the compound? The percent by mass of element Y?

A system at equilibrium is placed under stress by adding more reactant. If this reaction has a small equilibrium constant (Keq), how will the addition of this stress affect the equilibrium of this system.

Shown below is one step in the synthesis of racemic trans-2 - (1-propenyl) cyclohexanol from cyclohexane. from the list of reagents provided, choose the reagent (s) that will bring about this transformation.

The first shell can hold a maximum of two electrons. how does this explains the valence of hydrogen?

What is the number of proton in chlorine?

If you have 60 grams of MgCl2 in 5 liters of solution, what is the molarity of this solution?

Is evaporation/condensation a physical or chemical change? Explain

Would the two as yet undiscovered elements A+1 and D+2 react?

Ammonium perchlorate is a powerful solid rocket fuel, used in the Space Shuttle boosters. It decomposes into nitrogen gas, chlorine gas, oxygen gas and water vapor, releasing a great deal of energy.

Determine the number of bonding electrons and the number of nonbonding electrons in the structure of SeCl2.