Introduction:
When hydrogen and oxygen react, they form water. In this case, 4 grams of hydrogen and 4 grams of oxygen are ignited, and we need to determine the weight of water formed.

Explanation:
To find the weight of water formed, we can use the concept of stoichiometry. Stoichiometry is a branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions.

Step 1: Write the balanced chemical equation:
The balanced chemical equation for the reaction between hydrogen and oxygen to form water is:

2H₂ + O₂ → 2H₂O

This equation tells us that 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water.

Step 2: Calculate the number of moles:
To determine the number of moles of hydrogen and oxygen, we can use the formula:

Number of moles = Mass / Molar mass

The molar mass of hydrogen (H₂) is 2 g/mol, and the molar mass of oxygen (O₂) is 32 g/mol.

Number of moles of hydrogen = 4 g / 2 g/mol = 2 moles
Number of moles of oxygen = 4 g / 32 g/mol = 0.125 moles

Step 3: Determine the limiting reactant:
The limiting reactant is the one that is completely consumed in a reaction and determines the maximum amount of product that can be formed. To find the limiting reactant, we compare the moles of hydrogen and oxygen.

From the balanced equation, we can see that 2 moles of hydrogen react with 1 mole of oxygen. This means that 2 moles of hydrogen require 1 mole of oxygen for complete reaction. However, we only have 0.125 moles of oxygen, which is less than the required amount. Therefore, oxygen is the limiting reactant.

Step 4: Calculate the moles of water formed:
Since oxygen is the limiting reactant, we can calculate the moles of water formed using the mole ratio from the balanced equation:

1 mole of oxygen reacts to form 2 moles of water

Number of moles of water formed = 0.125 moles of oxygen × (2 moles of water / 1 mole of oxygen) = 0.25 moles

Step 5: Calculate the weight of water formed:
To calculate the weight of water formed, we multiply the number of moles of water by its molar mass:

Weight of water formed = Number of moles of water formed × Molar mass of water

The molar mass of water (H₂O) is 18 g/mol.

Weight of water formed = 0.25 moles × 18 g/mol = 4.5 grams

Conclusion:
Therefore, when 4 grams of hydrogen and 4 grams of oxygen are ignited, the weight of water formed is 4.5 grams.