Four solutions of unknown NaOH concentration are titrated with solutions of HCl. The following table lists the volume of each unknown NaOH solution, the volume of HCl solution required to reach the equivalence point, and the concentration of each HCl solution. Calculate the concentration (in M) of the unknown NaOH solution in each case.

NaOH Volume (mL) |
HCl Volume (mL) |
[HCl] (M) |

(a) 5.00 mL |
9.77 mL |
0.1599 M |

(b) 15.00 mL |
11.34 mL |
0.1311 M |

(c) 10.00 mL |
10.55 mL |
0.0889 M |

(d) 30.00 mL |
36.18 mL |
0.1021 |