For a mass concentration

Example: prepare a 100 mL solution of 1 mg/L of Na⁺ with NaCl salt

1 mg NaCl ≠ 1 mg Na⁺

M_NaCl = 58.44 g/mol ; M_Na = 22.99 g/mol ; M_Cl = 35.45 g/mol 

It can be seen here that in 1 mg of NaCl, there is in fact only 0.39 mg of Na⁺. This ratio must therefore be taken into account when calculating the mass of salt to be weighed in order to obtain a concentration of 1mg/L of Na⁺.

Where
m_salt = mass of salt to be weighed (in g)
M_salt = molar mass of salt (in g/mol)
M_element = molar mass of the element to consider (in g/mol)
C_f = final mass concentration (in g/L)
V_f = final volume (in L)

Thus in order to obtain a 100 mL solution of 1mg/L of Na⁺, we will need to weigh 0.254 mg of salt. In reality, weighing 0.254 mg of salt is practically impossible because the most precise scales have a precision of 0.1 mg. We will need to prepare a more concentrated solution and then dilute it in order to obtain the desired concentration level of Na⁺.

For a molar concentration

Example: prepare a 100 mL solution of 1 mol/L Na⁺ with Na₂SO₄ salt

Pay attention to the ratio between the number of moles of salt and of ion.
Here 1 mole of Na₂SO₄ contains 2 moles of Na⁺

Where
m_salt = mass of salt to be weighed (in g)
M_salt = molar mass of salt (in g/mol)
n_salt/n_element = molar ratio between the element and the salt
C_f = final molar concentration (in mol/L)
V_f = final volume (in L)

Thus, the mass of salt to be weighed in order to prepare a 1 mol/L solution of Na⁺ is :