how to write a conversion factor that relates moles of HCL to moles of NiCO3 to the reaction of hydrochloric acid reacts with some reacts with solid nickel carbonate

See below.

Step-by-step explanation:

We call this conversion factor the molar ratio.

1. Start with the balanced equation

NiCO₃ + 2HCl ⟶ NiCl₂ + H₂O + CO₂

2. Note the coefficients of the substances

The equation tells you that 1 mol of NiCO₃ reacts with 2 mol of HCl·

Thus, the conversion factor is either (1 mol NiCO₃/2 mol HCl) or (2 mol HCl/1 mol NiCO₃).

You choose the one that gives the correct units for the answer.

For example, if the question was, "How many moles of HCl will react with x mol of NiCO₃," you would write

x mol NiCO₃ * (2 mol HCl/1 mol NiCO₃) ⟶ 2x mol HCl

Notice that putting "mol HCl" in the numerator and "mol NiCO₃" in the denominator cancels "mol NiCO₃" and gives you the desired units of "mol HCl" in the answer.