Calculate [H3O+] for pH 1.86. Steps please.

[H₃O⁺] = [H⁺] = 10^-pH = 10⁻¹°⁸⁶ = 0.0138M in [H⁺]  

Explanation:

By definition pH = -log[H⁺] => [H⁺] = 10^-pH = 10⁻¹°⁸⁶ = 0.0138M in [H⁺]

Using your calculator ... I am using a TI-30XA scientific calculator.

Depending on the calculator, the answer may show as 0.0138, or  1.38x10⁻², or 1.38E-2 (=1.38 x 10⁻²). It is the user's job to insert dimensional units into answer of interest =>  0.0138M, or  1.38 x 10⁻²M, or 1.38E-2M.

1.38E-2 which is 1.38 x 10⁻².

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the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.