Arrange the following:
HOF, HOCl, HOBr, HOI in order of increasing acid strength.

Explanation

To arrange the compounds HOF, HOCl, HOBr, and HOI in order of increasing acid strength, we need to consider the electronegativity of the halogens (F, Cl, Br, I) attached to the hydroxyl group (OH). The higher the electronegativity of the halogen, the stronger the acid, as it will attract the electron pair in the O-H bond more effectively, making it easier to release the H⁺ ion.

### Step-by-Step Solution:

1. **Identify the Halogens and Their Electronegativity:**
- The compounds contain the following halogens: F (Fluorine), Cl (Chlorine), Br (Bromine), and I (Iodine).
- The electronegativity order is:
- F > Cl > Br > I

2. **Determine the Effect of Electronegativity on Acid Strength:**
- The more electronegative the halogen, the more it pulls the electron density away from the O-H bond, weakening it. This makes it easier for the compound to release H⁺ ions, thus increasing its acidity.

3. **Rank the Compounds Based on Electronegativity:**
- **HOF (Fluoroacetic acid)**: Fluorine is the most electronegative, making HOF the strongest acid.
- **HOCl (Chloroacetic acid)**: Chlorine is less electronegative than fluorine, so HOCl is less acidic than HOF.
- **HOBr (Bromoacetic acid)**: Bromine is less electronegative than chlorine, making HOBr less acidic than HOCl.
- **HOI (Iodoacetic acid)**: Iodine is the least electronegative, making HOI the weakest acid.

4. **Write the Final Order of Increasing Acid Strength:**
- Based on the analysis, the order of increasing acid strength is:
- HOI < HOBr < HOCl < HOF

### Final Answer:
The order of increasing acid strength is:
**HOI < HOBr < HOCl < HOF**