A flask contains three times as many moles of H2 gas as it does O2 gas. If hydrogen and oxygen are the only gases present, what is the total pressure in the flask if the partial pressure due to oxygen is P?

Answer

To solve the problem, we will follow these steps:

### Step 1: Understand the relationship between moles of gases
We know from the problem that the flask contains three times as many moles of H2 gas as it does O2 gas. If we denote the number of moles of O2 as \( n \), then the number of moles of H2 will be \( 3n \).

### Step 2: Apply Avogadro's Law
According to Avogadro's law, equal volumes of gases at the same temperature and pressure contain the same number of molecules. Therefore, the partial pressure of a gas is directly proportional to the number of moles of that gas.

### Step 3: Determine the partial pressures
Given that the partial pressure of O2 is \( P \), we can express the partial pressure of H2. Since there are three times as many moles of H2 as there are of O2, the partial pressure of H2 will be three times that of O2:
\[ P_{H2} = 3 \times P = 3P \]

### Step 4: Use Dalton's Law of Partial Pressures
Dalton's Law states that the total pressure in a mixture of gases is equal to the sum of the partial pressures of each individual gas. Therefore, we can calculate the total pressure \( P_{total} \) in the flask as follows:
\[ P_{total} = P_{H2} + P_{O2} \]
Substituting the values we found:
\[ P_{total} = 3P + P = 4P \]

### Final Answer
Thus, the total pressure in the flask is:
\[ \boxed{4P} \]