1. Understand the Concept

* Formic acid (HCOOH) is a weak acid. This means it doesn't completely ionize in water. We need to use its Ka value to find the [H+].

* Ka (acid dissociation constant) is a measure of how much an acid ionizes in water. The lower the Ka, the weaker the acid.

2. The Chemical Reaction

Formic acid dissociates in water according to this equilibrium:

HCOOH(aq) ⇌ H+(aq) + HCOO-(aq)

3. Setting Up the ICE Table

We'll use an ICE (Initial, Change, Equilibrium) table to calculate the equilibrium concentrations:

| | HCOOH | H+ | HCOO- |

|-------------|--------|-------|--------|

| Initial | 0.35 M | 0 | 0 |

| Change | -x | +x | +x |

| Equilibrium | 0.35-x| x | x |

4. The Ka Expression

The Ka expression for formic acid is:

Ka = [H+][HCOO-] / [HCOOH]

The Ka value for formic acid is 1.8 x 10^-4

5. Solve for x

Substitute the equilibrium concentrations from the ICE table into the Ka expression:

1.8 x 10^-4 = (x)(x) / (0.35 - x)

Since Ka is small, we can assume x is much smaller than 0.35, so we can simplify:

1.8 x 10^-4 = x^2 / 0.35

Solve for x:

x^2 = 6.3 x 10^-5

x = 0.0079 M

6. The Answer

Since x represents the concentration of H+ ions:

[H+] = 0.0079 M

Important Note: This answer is an approximation. For more accurate results, the quadratic equation should be used to solve for x.