If you spill some of the acid sample while transferring it to the erlenmeyer flask, then it affects the value of NaOH as it is less available in the flask.
Concentration of any species tells about the amount of that species present in given sample.
During the transferring of acid sample from any beaker to the erlenmeyer flask where already NaOH is present, some quantity of the acid spills out. Then we don't get the exact value of the required concentration of NaOH after mixing as not all the quantities of acid will pour on the flask.
Hence due to less neutralization, concentration of NaOH will affect.
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